ecliptic – Cosmic Reflections

Zodiacal Light 2023

Zodiacal Light 2022

Zodiacal Light 2021

Zodiacal Light 2020

Comet Orbital Elements

Zodiacal Light 2019

Like Sun, Like Moon

The Zodiacal Light

In 2023, the best dates and times for observing the zodiacal light are listed in the calendar below. The sky must be very clear with little or no light pollution. The specific times listed are for Tucson, Arizona (32° 16′ N, 111° 03′ W).

Here’s a nicely-formatted printable PDF file of the zodiacal light calendar:

Zodiacal Light Calendar 2023 Download

January 2023

SUN	MON	TUE	WED	THU	FRI	SAT
1	2	3	4	5	6	7
8 Zodiacal Light 7:02 – 7:14 p.m. West	9 Zodiacal Light 7:03 – 8:03 p.m. West	10 Zodiacal Light 7:04 – 8:04 p.m. West	11 Zodiacal Light 7:04 – 8:04 p.m. West	12 Zodiacal Light 7:05 – 8:05 p.m. West	13 Zodiacal Light 7:06 – 8:06 p.m. West	14 Zodiacal Light 7:07 – 8:07 p.m. West
15 Zodiacal Light 7:07 – 8:07 p.m. West	16 Zodiacal Light 7:08 – 8:08 p.m. West	17 Zodiacal Light 7:09 – 8:09 p.m. West	18 Zodiacal Light 7:10 – 8:10 p.m. West	19 Zodiacal Light 7:11 – 8:11 p.m. West	20 Zodiacal Light 7:11 – 8:11 p.m. West	21 Zodiacal Light 7:12 – 8:12 p.m. West
22 Zodiacal Light 7:13 – 8:13 p.m. West	23	24	25	26	27	28
29	30	31

February 2023

SUN	MON	TUE	WED	THU	FRI	SAT
			1	2	3	4
5	6	7 Zodiacal Light 7:26 – 7:58 p.m. West	8 Zodiacal Light 7:27 – 8:27 p.m. West	9 Zodiacal Light 7:27 – 8:27 p.m. West	10 Zodiacal Light 7:28 – 8:28 p.m. West	11 Zodiacal Light 7:29 – 8:29 p.m. West
12 Zodiacal Light 7:30 – 8:30 p.m. West	13 Zodiacal Light 7:31 – 8:31 p.m. West	14 Zodiacal Light 7:31 – 8:31 p.m. West	15 Zodiacal Light 7:32 – 8:32 p.m. West	16 Zodiacal Light 7:33 – 8:33 p.m. West	17 Zodiacal Light 7:34 – 8:34 p.m. West	18 Zodiacal Light 7:34 – 8:34 p.m. West
19 Zodiacal Light 7:35 – 8:35 p.m. West	20 Zodiacal Light 7:36 – 8:36 p.m. West	21	22	23	24	25
26	27	28

March 2023

SUN	MON	TUE	WED	THU	FRI	SAT
			1	2	3	4
5	6	7	8	9 Zodiacal Light 7:49 – 8:43 p.m. West	10 Zodiacal Light 7:49 – 8:49 p.m. West	11 Zodiacal Light 7:50 – 8:50 p.m. West
12 Zodiacal Light 7:51 – 8:51 p.m. West	13 Zodiacal Light 7:52 – 8:52 p.m. West	14 Zodiacal Light 7:52 – 8:52 p.m. West	15 Zodiacal Light 7:53 – 8:53 p.m. West	16 Zodiacal Light 7:54 – 8:54 p.m. West	17 Zodiacal Light 7:55 – 8:55 p.m. West	18 Zodiacal Light 7:55 – 8:55 p.m. West
19 Zodiacal Light 7:56 – 8:56 p.m. West	20 Zodiacal Light 7:57 – 8:57 p.m. West	21 Zodiacal Light 7:58 – 8:58 p.m. West	22	23	24	25
26	27	28	29	30	31

April 2023

SUN	MON	TUE	WED	THU	FRI	SAT
						1
2	3	4	5	6	7 Zodiacal Light 8:12 – 8:37 p.m. West	8 Zodiacal Light 8:13 – 9:13 p.m. West
9 Zodiacal Light 8:14 – 9:14 p.m. West	10 Zodiacal Light 8:14 – 9:14 p.m. West	11 Zodiacal Light 8:15 – 9:15 p.m. West	12 Zodiacal Light 8:16 – 9:16 p.m. West	13 Zodiacal Light 8:17 – 9:17 p.m. West	14 Zodiacal Light 8:18 – 9:18 p.m. West	15 Zodiacal Light 8:19 – 9:19 p.m. West
16 Zodiacal Light 8:20 – 9:20 p.m. West	17 Zodiacal Light 8:21 – 9:21 p.m. West	18 Zodiacal Light 8:22 – 9:22 p.m. West	19 Zodiacal Light 8:23 – 9:23 p.m. West	20 Zodiacal Light 8:24 – 9:24 p.m. West	21	22
23	24	25	26	27	28	29
30

May 2023

SUN	MON	TUE	WED	THU	FRI	SAT
	1	2	3	4	5	6
7 Zodiacal Light 8:41 – 9:41 p.m. West	8 Zodiacal Light 8:42 – 9:42 p.m. West	9 Zodiacal Light 8:43 – 9:43 p.m. West	10 Zodiacal Light 8:44 – 9:44 p.m. West	11 Zodiacal Light 8:45 – 9:45 p.m. West	12 Zodiacal Light 8:46 – 9:46 p.m. West	13 Zodiacal Light 8:47 – 9:47 p.m. West
14 Zodiacal Light 8:48 – 9:48 p.m. West	15 Zodiacal Light 8:49 – 9:49 p.m. West	16 Zodiacal Light 8:50 – 9:50 p.m. West	17 Zodiacal Light 8:51 – 9:51 p.m. West	18 Zodiacal Light 8:52 – 9:52 p.m. West	19 Zodiacal Light 8:53 – 9:53 p.m. West	20 Zodiacal Light 8:54 – 9:54 p.m. West
21	22	23	24	25	26	27
28	29	30	31

June 2023

July 2023

August 2023

SUN	MON	TUE	WED	THU	FRI	SAT
		1	2	3	4	5
6	7	8	9	10	11	12
13	14 Zodiacal Light 3:59 – 4:18 a.m. East	15 Zodiacal Light 3:19 – 4:19 a.m. East	16 Zodiacal Light 3:20 – 4:20 a.m. East	17 Zodiacal Light 3:21 – 4:21 a.m. East	18 Zodiacal Light 3:22 – 4:22 a.m. East	19 Zodiacal Light 3:23 – 4:23 a.m. East
20 Zodiacal Light 3:24 – 4:24 a.m. East	21 Zodiacal Light 3:24 – 4:24 a.m. East	22 Zodiacal Light 3:25 – 4:25 a.m. East	23 Zodiacal Light 3:26 – 4:26 a.m. East	24 Zodiacal Light 3:27 – 4:27 a.m. East	25 Zodiacal Light 3:28 – 4:28 a.m. East	26 Zodiacal Light 3:29 – 4:29 a.m. East
27 Zodiacal Light 3:30 – 4:30 a.m. East	28 Zodiacal Light 3:31 – 4:31 a.m. East	29 Zodiacal Light 4:03 – 4:31 a.m. East	30	31

September 2023

SUN	MON	TUE	WED	THU	FRI	SAT
					1	2
3	4	5	6	7	8	9
10	11	12	13 Zodiacal Light 3:43 – 4:43 a.m. East	14 Zodiacal Light 3:44 – 4:44 a.m. East	15 Zodiacal Light 3:45 – 4:45 a.m. East	16 Zodiacal Light 3:46 – 4:46 a.m. East
17 Zodiacal Light 3:46 – 4:46 a.m. East	18 Zodiacal Light 3:47 – 4:47 a.m. East	19 Zodiacal Light 3:48 – 4:48 a.m. East	20 Zodiacal Light 3:49 – 4:49 a.m. East	21 Zodiacal Light 3:49 – 4:49 a.m. East	22 Zodiacal Light 3:50 – 4:50 a.m. East	23 Zodiacal Light 3:51 – 4:51 a.m. East
24 Zodiacal Light 3:51 – 4:51 a.m. East	25 Zodiacal Light 3:52 – 4:52 a.m. East	26 Zodiacal Light 3:53 – 4:53 a.m. East	27 Zodiacal Light 4:06 – 4:53 a.m. East	28	29	30

October 2023

SUN	MON	TUE	WED	THU	FRI	SAT
1	2	3	4	5	6	7
8	9	10	11	12	13 Zodiacal Light 4:04 – 5:04 a.m. East	14 Zodiacal Light 4:05 – 5:05 a.m. East
15 Zodiacal Light 4:06 – 5:06 a.m. East	16 Zodiacal Light 4:06 – 5:06 a.m. East	17 Zodiacal Light 4:07 – 5:07 a.m. East	18 Zodiacal Light 4:08 – 5:08 a.m. East	19 Zodiacal Light 4:08 – 5:08 a.m. East	20 Zodiacal Light 4:09 – 5:09 a.m. East	21 Zodiacal Light 4:10 – 5:10 a.m. East
22 Zodiacal Light 4:11 – 5:11 a.m. East	23 Zodiacal Light 4:11 – 5:11 a.m. East	24 Zodiacal Light 4:12 – 5:12 a.m. East	25 Zodiacal Light 4:13 – 5:13 a.m. East	26 Zodiacal Light 4:13 – 5:13 a.m. East	27	28
29	30	31

November 2023

SUN	MON	TUE	WED	THU	FRI	SAT
			1	2	3	4
5	6	7	8	9	10	11
12 Zodiacal Light 4:26 – 5:26 a.m. East	13 Zodiacal Light 4:27 – 5:27 a.m. East	14 Zodiacal Light 4:27 – 5:27 a.m. East	15 Zodiacal Light 4:28 – 5:28 a.m. East	16 Zodiacal Light 4:29 – 5:29 a.m. East	17 Zodiacal Light 4:30 – 5:30 a.m. East	18 Zodiacal Light 4:30 – 5:30 a.m. East
19 Zodiacal Light 4:31 – 5:31 a.m. East	20 Zodiacal Light 4:32 – 5:32 a.m. East	21 Zodiacal Light 4:33 – 5:33 a.m. East	22 Zodiacal Light 4:33 – 5:33 a.m. East	23 Zodiacal Light 4:34 – 5:34 a.m. East	24 Zodiacal Light 4:35 – 5:35 a.m. East	25 Zodiacal Light 5:13 – 5:36 a.m. East
26	27	28	29	30

December 2023

SUN	MON	TUE	WED	THU	FRI	SAT
					1	2
3	4	5	6	7	8	9
10	11 Zodiacal Light 4:47 – 5:47 a.m. East	12 Zodiacal Light 4:48 – 5:48 a.m. East	13 Zodiacal Light 4:48 – 5:48 a.m. East	14 Zodiacal Light 4:49 – 5:49 a.m. East	15 Zodiacal Light 4:50 – 5:50 a.m. East	16 Zodiacal Light 4:50 – 5:50 a.m. East
17 Zodiacal Light 4:51 – 5:51 a.m. East	18 Zodiacal Light 4:51 – 5:51 a.m. East	19 Zodiacal Light 4:52 – 5:52 a.m. East	20 Zodiacal Light 4:53 – 5:53 a.m. East	21 Zodiacal Light 4:53 – 5:53 a.m. East	22 Zodiacal Light 4:54 – 5:54 a.m. East	23 Zodiacal Light 4:54 – 5:54 a.m. East
24 Zodiacal Light 5:14 – 5:54 a.m. East	25	26	27	28	29	30
31

The best nights to observe the zodiacal light at mid-northern latitudes occur when the ecliptic plane intersects the horizon at an angle of 60° or steeper. The dates above were chosen on that basis, with the Sun at least 18° below the horizon and the Moon below the horizon being used to calculate the times. An interval of time of one hour either before morning twilight or after evening twilight was chosen arbitrarily because it is the “best one hour” for observing the zodiacal light. The zodiacal light cone will be brightest and will reach highest above the horizon when the Sun is 18° below the horizon (astronomical twilight), but no less.

If you are interested in calculating the angle the ecliptic makes with your horizon for any date and time, you can use the following formula:

$\cos I = \cos \varepsilon \sin \phi-\sin \varepsilon \cos \phi \sin \theta$

where I is the angle between the ecliptic and the horizon, ε is  the obliquity of the ecliptic, φ is the latitude of the observer, and θ is the local sidereal time (the right ascension of objects on the observer's meridian at the time of observation).

Here’s a SAS program I wrote to do these calculations:

Zodiacal Light.sas Download

References
Meeus, J. Astronomical Algorithms. 2nd ed., Willmann-Bell, 1998, p. 99.

In 2022, the best dates and times for observing the zodiacal light are listed in the calendar below. The sky must be very clear with little or no light pollution. The specific times listed are for Dodgeville, Wisconsin (42° 58′ N, 90° 08′ W).

Here’s a nicely-formatted printable PDF file of the zodiacal light calendar:

Zodiacal Light Calendar 2022 Download

January 2022

SUN	MON	TUE	WED	THU	FRI	SAT
						1
2	3	4	5	6	7	8
9	10	11	12	13	14	15
16	17	18	19 Zodiacal Light 6:36 – 6:38 p.m. West	20 Zodiacal Light 6:37 – 7:37 p.m. West	21 Zodiacal Light 6:38 – 7:38 p.m. West	22 Zodiacal Light 6:39 – 7:39 p.m. West
23 Zodiacal Light 6:40 – 7:40 p.m. West	24 Zodiacal Light 6:41 – 7:41 p.m. West	25 Zodiacal Light 6:42 – 7:42 p.m. West	26 Zodiacal Light 6:43 – 7:43 p.m. West	27 Zodiacal Light 6:44 – 7:44 p.m. West	28 Zodiacal Light 6:46 – 7:46 p.m. West	29 Zodiacal Light 6:47 – 7:47 p.m. West
30 Zodiacal Light 6:48 – 7:48 p.m. West	31 Zodiacal Light 6:49 – 7:49 p.m. West

February 2022

SUN	MON	TUE	WED	THU	FRI	SAT
		1 Zodiacal Light 6:50 – 7:50 p.m. West	2 Zodiacal Light 7:04 – 7:51 p.m. West	3	4	5
6	7	8	9	10	11	12
13	14	15	16	17	18 Zodiacal Light 7:10 – 7:51 p.m. West	19 Zodiacal Light 7:12 – 8:12 p.m. West
20 Zodiacal Light 7:13 – 8:13 p.m. West	21 Zodiacal Light 7:14 – 8:14 p.m. West	22 Zodiacal Light 7:15 – 8:15 p.m. West	23 Zodiacal Light 7:16 – 8:16 p.m. West	24 Zodiacal Light 7:18 – 8:18 p.m. West	25 Zodiacal Light 7:19 – 8:19 p.m. West	26 Zodiacal Light 7:20 – 8:20 p.m. West
27 Zodiacal Light 7:21 – 8:21 p.m. West	28 Zodiacal Light 7:22 – 8:22 p.m. West

March 2022

SUN	MON	TUE	WED	THU	FRI	SAT
		1 Zodiacal Light 7:24 – 8:24 p.m. West	2 Zodiacal Light 7:25 – 8:25 p.m. West	3 Zodiacal Light 7:26 – 8:26 p.m. West	4 Zodiacal Light 8:14 – 8:27 p.m. West	5
6	7	8	9	10	11	12
13	14	15	16	17	18	19 Zodiacal Light 8:47 – 8:59 p.m. West
20 Zodiacal Light 8:48 – 9:48 p.m. West	21 Zodiacal Light 8:49 – 9:49 p.m. West	22 Zodiacal Light 8:51 – 9:51 p.m. West	23 Zodiacal Light 8:52 – 9:52 p.m. West	24 Zodiacal Light 8:53 – 9:53 p.m. West	25 Zodiacal Light 8:55 – 9:55 p.m. West	26 Zodiacal Light 8:56 – 9:56 p.m. West
27 Zodiacal Light 8:57 – 9:57 p.m. West	28 Zodiacal Light 8:59 – 9:59 p.m. West	29 Zodiacal Light 9:00 – 10:00 p.m. West	30 Zodiacal Light 9:02 – 10:02 p.m. West	31 Zodiacal Light 9:03 – 10:03 p.m. West

April 2022

May 2022

June 2022

July 2022

August 2022

September 2022

SUN	MON	TUE	WED	THU	FRI	SAT
				1 Zodiacal Light 3:44 – 4:44 a.m. East	2 Zodiacal Light 3:45 – 4:45 a.m. East	3 Zodiacal Light 3:47 – 4:47 a.m. East
4 Zodiacal Light 3:48 – 4:48 a.m. East	5 Zodiacal Light 3:49 – 4:49 a.m. East	6 Zodiacal Light 3:51 – 4:51 a.m. East	7 Zodiacal Light 3:52 – 4:52 a.m. East	8 Zodiacal Light 3:57 – 4:54 a.m. East	9	10
11	12	13	14	15	16	17
18	19	20	21	22	23	24 Zodiacal Light 4:14 – 5:14 a.m. East
25 Zodiacal Light 4:16 – 5:16 a.m. East	26 Zodiacal Light 4:17 – 5:17 a.m. East	27 Zodiacal Light 4:18 – 5:18 a.m. East	28 Zodiacal Light 4:19 – 5:19 a.m. East	29 Zodiacal Light 4:21 – 5:21 a.m. East	30 Zodiacal Light 4:22 – 5:22 a.m. East

October 2022

SUN	MON	TUE	WED	THU	FRI	SAT
						1 Zodiacal Light 4:23 – 5:23 a.m. East
2 Zodiacal Light 4:24 – 5:24 a.m. East	3 Zodiacal Light 4:25 – 5:25 a.m. East	4 Zodiacal Light 4:27 – 5:27 a.m. East	5 Zodiacal Light 4:28 – 5:28 a.m. East	6 Zodiacal Light 4:29 – 5:29 a.m. East	7 Zodiacal Light 4:30 – 5:30 a.m. East	8 Zodiacal Light 5:28 – 5:31 a.m. East
9	10	11	12	13	14	15
16	17	18	19	20	21	22
23 Zodiacal Light 4:48 – 5:12 a.m. East	24 Zodiacal Light 4:50 – 5:50 a.m. East	25 Zodiacal Light 4:51 – 5:51 a.m. East	26 Zodiacal Light 4:52 – 5:52 a.m. East	27 Zodiacal Light 4:53 – 5:53 a.m. East	28 Zodiacal Light 4:54 – 5:54 a.m. East	29 Zodiacal Light 4:55 – 5:55 a.m. East
30 Zodiacal Light 4:56 – 5:56 a.m. East	31 Zodiacal Light 4:57 – 5:57 a.m. East

November 2022

SUN	MON	TUE	WED	THU	FRI	SAT
		1 Zodiacal Light 4:59 – 5:59 a.m. East	2 Zodiacal Light 5:00 – 6:00 a.m. East	3 Zodiacal Light 5:01 – 6:01 a.m. East	4 Zodiacal Light 5:02 – 6:02 a.m. East	5 Zodiacal Light 5:03 – 6:03 a.m. East
6 Zodiacal Light 4:35 – 5:04 a.m. East	7	8	9	10	11	12
13	14	15	16	17	18	19
20	21	22 Zodiacal Light 4:21 – 5:21 a.m. East	23 Zodiacal Light 4:22 – 5:22 a.m. East	24 Zodiacal Light 4:23 – 5:23 a.m. East	25 Zodiacal Light 4:24 – 5:24 a.m. East	26 Zodiacal Light 4:25 – 5:25 a.m. East
27 Zodiacal Light 4:26 – 5:26 a.m. East	28 Zodiacal Light 4:27 – 5:27 a.m. East	29 Zodiacal Light 4:28 – 5:28 a.m. East	30 Zodiacal Light 4:29 – 5:29 a.m. East

December 2022

If you are interested in calculating the angle the ecliptic makes with your horizon for any date and time, you can use the following formula:

$\cos I = \cos \varepsilon \sin \phi-\sin \varepsilon \cos \phi \sin \theta$

where I is the angle between the ecliptic and the horizon, ε is  the obliquity of the ecliptic, φ is the latitude of the observer, and θ is the local sidereal time (the right ascension of objects on the observer's meridian at the time of observation).

Here’s a SAS program I wrote to do these calculations:

Zodiacal Light.sas Download

References
Meeus, J. Astronomical Algorithms. 2nd ed., Willmann-Bell, 1998, p. 99.

In 2021, the best dates and times for observing the zodiacal light are listed in the calendar below. The sky must be very clear with little or no light pollution. The specific times listed are for Dodgeville, Wisconsin (42° 58′ N, 90° 08′ W).

Here’s a nicely-formatted printable PDF file of the zodiacal light calendar:

Zodiacal Light Calendar 2021 Download

January 2021

February 2021

SUN	MON	TUE	WED	THU	FRI	SAT
	1 Zodiacal Light 6:51 – 7:51 p.m. West	2 Zodiacal Light 6:52 – 7:52 p.m. West	3 Zodiacal Light 6:53 – 7:53 p.m. West	4 Zodiacal Light 6:54 – 7:54 p.m. West	5 Zodiacal Light 6:56 – 7:56 p.m. West	6 Zodiacal Light 6:57 – 7:57 p.m. West
7 Zodiacal Light 6:58 – 7:58 p.m. West	8 Zodiacal Light 6:59 – 7:59 p.m. West	9 Zodiacal Light 7:00 – 8:00 p.m. West	10 Zodiacal Light 7:02 – 8:02 p.m. West	11 Zodiacal Light 7:03 – 8:03 p.m. West	12 Zodiacal Light 7:04 – 8:04 p.m. West	13
14	15	16	17	18	19	20
21	22	23	24	25	26	27
28 Zodiacal Light 7:23 – 7:36 p.m. West

March 2021

SUN	MON	TUE	WED	THU	FRI	SAT
	1 Zodiacal Light 7:25 – 8:25 p.m. West	2 Zodiacal Light 7:26 – 8:26 p.m. West	3 Zodiacal Light 7:27 – 8:27 p.m. West	4 Zodiacal Light 7:28 – 8:28 p.m. West	5 Zodiacal Light 7:29 – 8:29 p.m. West	6 Zodiacal Light 7:31 – 8:31 p.m. West
7 Zodiacal Light 7:32 – 8:32 p.m. West	8 Zodiacal Light 7:33 – 8:33 p.m. West	9 Zodiacal Light 7:34 – 8:34 p.m. West	10 Zodiacal Light 7:36 – 8:36 p.m. West	11 Zodiacal Light 7:37 – 8:37 p.m. West	12 Zodiacal Light 7:38 – 8:38 p.m. West	13 Zodiacal Light 7:40 – 8:40 p.m. West
14 Zodiacal Light 8:41 – 9:41 p.m. West	15	16	17	18	19	20
21	22	23	24	25	26	27
28	29	30 Zodiacal Light 9:03 – 10:03 p.m. West	31 Zodiacal Light 9:04 – 10:04 p.m. West

April 2021

SUN	MON	TUE	WED	THU	FRI	SAT
				1 Zodiacal Light 9:05 – 10:05 p.m. West	2 Zodiacal Light 9:07 – 10:07 p.m. West	3 Zodiacal Light 9:08 – 10:08 p.m. West
4 Zodiacal Light 9:10 – 10:10 p.m. West	5 Zodiacal Light 9:11 – 10:11 p.m. West	6 Zodiacal Light 9:13 – 10:13 p.m. West	7 Zodiacal Light 9:14 – 10:14 p.m. West	8 Zodiacal Light 9:16 – 10:16 p.m. West	9 Zodiacal Light 9:17 – 10:17 p.m. West	10 Zodiacal Light 9:19 – 10:19 p.m. West
11 Zodiacal Light 9:20 – 10:20 p.m. West	12 Zodiacal Light 9:22 – 10:22 p.m. West	13 Zodiacal Light 9:24 – 10:24 p.m. West	14	15	16	17
18	19	20	21	22	23	24
25	26	27	28	29	30

May 2021

June 2021

July 2021

August 2021

September 2021

SUN	MON	TUE	WED	THU	FRI	SAT
			1	2	3	4
5	6 Zodiacal Light 3:52 – 4:52 a.m. East	7 Zodiacal Light 3:53 – 4:53 a.m. East	8 Zodiacal Light 3:54 – 4:54 a.m. East	9 Zodiacal Light 3:56 – 4:56 a.m. East	10 Zodiacal Light 3:57 – 4:57 a.m. East	11 Zodiacal Light 3:58 – 4:58 a.m. East
12 Zodiacal Light 4:00 – 5:00 a.m. East	13 Zodiacal Light 4:01 – 5:01 a.m. East	14 Zodiacal Light 4:02 – 5:02 a.m. East	15 Zodiacal Light 4:04 – 5:04 a.m. East	16 Zodiacal Light 4:05 – 5:05 a.m. East	17 Zodiacal Light 4:06 – 5:06 a.m. East	18 Zodiacal Light 4:08 – 5:08 a.m. East
19 Zodiacal Light 4:59 – 5:09 a.m. East	20	21	22	23	24	25
26	27	28	29	30

October 2021

SUN	MON	TUE	WED	THU	FRI	SAT
					1	2
3	4	5 Zodiacal Light 4:28 – 5:28 a.m. East	6 Zodiacal Light 4:30 – 5:30 a.m. East	7 Zodiacal Light 4:31 – 5:31 a.m. East	8 Zodiacal Light 4:32 – 5:32 a.m. East	9 Zodiacal Light 4:33 – 5:33 a.m. East
10 Zodiacal Light 4:34 – 5:34 a.m. East	11 Zodiacal Light 4:35 – 5:35 a.m. East	12 Zodiacal Light 4:37 – 5:37 a.m. East	13 Zodiacal Light 4:38 – 5:38 a.m. East	14 Zodiacal Light 4:39 – 5:39 a.m. East	15 Zodiacal Light 4:40 – 5:40 a.m. East	16 Zodiacal Light 4:41 – 5:41 a.m. East
17 Zodiacal Light 4:42 – 5:42 a.m. East	18 Zodiacal Light 5:03 – 5:43 a.m. East	19	20	21	22	23
24	25	26	27	28	29	30
31

November 2021

SUN	MON	TUE	WED	THU	FRI	SAT
	1	2	3	4 Zodiacal Light 5:03 – 6:03 a.m. East	5 Zodiacal Light 5:04 – 6:04 a.m. East	6 Zodiacal Light 5:05 – 6:05 a.m. East
7 Zodiacal Light 4:06 – 5:06 a.m. East	8 Zodiacal Light 4:07 – 5:07 a.m. East	9 Zodiacal Light 4:08 – 5:08 a.m. East	10 Zodiacal Light 4:09 – 5:09 a.m. East	11 Zodiacal Light 4:10 – 5:10 a.m. East	12 Zodiacal Light 4:12 – 5:12 a.m. East	13 Zodiacal Light 4:13 – 5:13 a.m. East
14 Zodiacal Light 4:14 – 5:14 a.m. East	15 Zodiacal Light 4:15 – 5:15 a.m. East	16 Zodiacal Light 4:16 – 5:16 a.m. East	17 Zodiacal Light 5:06 – 5:17 a.m. East	18	19	20
21	22	23	24	25	26	27
28	29	30

December 2021

If you are interested in calculating the angle the ecliptic makes with your horizon for any date and time, you can use the following formula:

$\cos I = \cos \varepsilon \sin \phi-\sin \varepsilon \cos \phi \sin \theta$

where I is the angle between the ecliptic and the horizon, ε is  the obliquity of the ecliptic, φ is the latitude of the observer, and θ is the local sidereal time (the right ascension of objects on the observer's meridian at the time of observation).

Here’s a SAS program I wrote to do these calculations:

Zodiacal Light.sas Download

References
Meeus, J. Astronomical Algorithms. 2nd ed., Willmann-Bell, 1998, p. 99.

In 2020, the best dates and times for observing the zodiacal light are listed in the calendar below. The sky must be very clear with little or no light pollution. The specific times listed are for Dodgeville, Wisconsin (42° 58′ N, 90° 08′ W).

Here’s a nicely-formatted printable PDF file of the zodiacal light calendar:

Zodiacal Light Calendar 2020 Download

January 2020

SUN	MON	TUE	WED	THU	FRI	SAT
			1	2	3	4
5	6	7	8	9	10	11
12 Zodiacal Light 6:28 – 7:07 p.m. West	13 Zodiacal Light 6:29 – 7:29 p.m. West	14 Zodiacal Light 6:30 – 7:30 p.m. West	15 Zodiacal Light 6:31 – 7:31 p.m. West	16 Zodiacal Light 6:32 – 7:32 p.m. West	17 Zodiacal Light 6:33 – 7:33 p.m. West	18 Zodiacal Light 6:34 – 7:34 p.m. West
19 Zodiacal Light 6:35 – 7:35 p.m. West	20 Zodiacal Light 6:36 – 7:36 p.m. West	21 Zodiacal Light 6:37 – 7:37 p.m. West	22 Zodiacal Light 6:38 – 7:38 p.m. West	23 Zodiacal Light 6:39 – 7:39 p.m. West	24 Zodiacal Light 6:41 – 7:41 p.m. West	25 Zodiacal Light 6:42 – 7:42 p.m. West
26	27	28	29	30	31

February 2020

SUN	MON	TUE	WED	THU	FRI	SAT
						1
2	3	4	5	6	7	8
9	10 Zodiacal Light 7:00 – 7:17 p.m. West	11 Zodiacal Light 7:01 – 8:01 p.m. West	12 Zodiacal Light 7:03 – 8:03 p.m. West	13 Zodiacal Light 7:04 – 8:04 p.m. West	14 Zodiacal Light 7:05 – 8:05 p.m. West	15 Zodiacal Light 7:06 – 8:06 p.m. West
16 Zodiacal Light 7:07 – 8:07 p.m. West	17 Zodiacal Light 7:09 – 8:09 p.m. West	18 Zodiacal Light 7:10 – 8:10 p.m. West	19 Zodiacal Light 7:11 – 8:11 p.m. West	20 Zodiacal Light 7:12 – 8:12 p.m. West	21 Zodiacal Light 7:13 – 8:13 p.m. West	22 Zodiacal Light 7:15 – 8:15 p.m. West
23 Zodiacal Light 7:16 – 8:16 p.m. West	24 Zodiacal Light 7:17 – 8:17 p.m. West	25	26	27	28	29

March 2020

SUN	MON	TUE	WED	THU	FRI	SAT
1	2	3	4	5	6	7
8	9	10	11 Zodiacal Light 8:37 – 9:37 p.m. West	12 Zodiacal Light 8:38 – 9:38 p.m. West	13 Zodiacal Light 8:39 – 9:39 p.m. West	14 Zodiacal Light 8:41 – 9:41 p.m. West
15 Zodiacal Light 8:42 – 9:42 p.m. West	16 Zodiacal Light 8:43 – 9:43 p.m. West	17 Zodiacal Light 8:45 – 9:45 p.m. West	18 Zodiacal Light 8:46 – 9:46 p.m. West	19 Zodiacal Light 8:47 – 9:47 p.m. West	20 Zodiacal Light 8:49 – 9:49 p.m. West	21 Zodiacal Light 8:50 – 9:50 p.m. West
22 Zodiacal Light 8:51 – 9:51 p.m. West	23 Zodiacal Light 8:53 – 9:53 p.m. West	24 Zodiacal Light 8:54 – 9:54 p.m. West	25 Zodiacal Light 8:55 – 9:55 p.m. West	26	27	28
29	30	31

April 2020

SUN	MON	TUE	WED	THU	FRI	SAT
			1	2	3	4
5	6	7	8	9 Zodiacal Light 9:17 – 9:51 p.m. West	10 Zodiacal Light 9:19 – 10:19 p.m. West	11 Zodiacal Light 9:20 – 10:20 p.m. West
12 Zodiacal Light 9:22 – 10:22 p.m. West	13 Zodiacal Light 9:23 – 10:23 p.m. West	14 Zodiacal Light 9:25 – 10:25 p.m. West	15 Zodiacal Light 9:27 – 10:27 p.m. West	16 Zodiacal Light 9:28 – 10:28 p.m. West	17 Zodiacal Light 9:30 – 10:30 p.m. West	18 Zodiacal Light 9:31 – 10:31 p.m. West
19 Zodiacal Light 9:33 – 10:33 p.m. West	20 Zodiacal Light 9:35 – 10:35 p.m. West	21 Zodiacal Light 9:36 – 10:36 p.m. West	22 Zodiacal Light 9:38 – 10:38 p.m. West	23 Zodiacal Light 9:40 – 10:40 p.m. West	24 Zodiacal Light 9:41 – 10:41 p.m. West	25
26	27	28	29	30

September 2020

SUN	MON	TUE	WED	THU	FRI	SAT
		1	2	3	4	5
6	7	8	9	10	11	12
13	14	15	16 Zodiacal Light 4:05 – 5:05 a.m. East	17 Zodiacal Light 4:06 – 5:06 a.m. East	18 Zodiacal Light 4:07 – 5:07 a.m. East	19 Zodiacal Light 4:09 – 5:09 a.m. East
20 Zodiacal Light 4:10 – 5:10 a.m. East	21 Zodiacal Light 4:11 – 5:11 a.m. East	22 Zodiacal Light 4:13 – 5:13 a.m. East	23 Zodiacal Light 4:14 – 5:14 a.m. East	24 Zodiacal Light 4:15 – 5:15 a.m. East	25 Zodiacal Light 4:16 – 5:16 a.m. East	26 Zodiacal Light 4:17 – 5:17 a.m. East
27 Zodiacal Light 4:19 – 5:19 a.m. East	28 Zodiacal Light 4:20 – 5:20 a.m. East	29 Zodiacal Light 4:27 – 5:21 a.m. East	30

October 2020

SUN	MON	TUE	WED	THU	FRI	SAT
				1	2	3
4	5	6	7	8	9	10
11	12	13	14	15	16 Zodiacal Light 4:41 – 5:41 a.m. East	17 Zodiacal Light 4:42 – 5:42 a.m. East
18 Zodiacal Light 4:43 – 5:43 a.m. East	19 Zodiacal Light 4:44 – 5:44 a.m. East	20 Zodiacal Light 4:46 – 5:46 a.m. East	21 Zodiacal Light 4:47 – 5:47 a.m. East	22 Zodiacal Light 4:48 – 5:48 a.m. East	23 Zodiacal Light 4:49 – 5:49 a.m. East	24 Zodiacal Light 4:50 – 5:50 a.m. East
25 Zodiacal Light 4:51 – 5:51 a.m. East	26 Zodiacal Light 4:52 – 5:52 a.m. East	27 Zodiacal Light 4:53 – 5:53 a.m. East	28 Zodiacal Light 4:55 – 5:55 a.m. East	29 Zodiacal Light 5:24 – 5:56 a.m. East	30	31

November 2020

SUN	MON	TUE	WED	THU	FRI	SAT
1	2	3	4	5	6	7
8	9	10	11	12	13	14 Zodiacal Light 4:13 – 5:13 a.m. East
15 Zodiacal Light 4:15 – 5:15 a.m. East	16 Zodiacal Light 4:16 – 5:16 a.m. East	17 Zodiacal Light 4:17 – 5:17 a.m. East	18 Zodiacal Light 4:18 – 5:18 a.m. East	19 Zodiacal Light 4:19 – 5:19 a.m. East	20 Zodiacal Light 4:20 – 5:20 a.m. East	21 Zodiacal Light 4:21 – 5:21 a.m. East
22 Zodiacal Light 4:22 – 5:22 a.m. East	23 Zodiacal Light 4:23 – 5:23 a.m. East	24 Zodiacal Light 4:24 – 5:24 a.m. East	25 Zodiacal Light 4:25 – 5:25 a.m. East	26 Zodiacal Light 4:26 – 5:26 a.m. East	27 Zodiacal Light 4:27 – 5:27 a.m. East	28 Zodiacal Light 5:17 – 5:28 a.m. East
29	30

If you are interested in calculating the angle the ecliptic makes with your horizon for any date and time, you can use the following formula:

$\cos I = \cos \varepsilon \sin \phi-\sin \varepsilon \cos \phi \sin \theta$

where I is the angle between the ecliptic and the horizon, ε is  the obliquity of the ecliptic, φ is the latitude of the observer, and θ is the local sidereal time (the right ascension of objects on the observer's meridian at the time of observation).

Here’s a SAS program I wrote to do these calculations:

Zodiacal Light.sas Download

References
Meeus, J. Astronomical Algorithms. 2nd ed., Willmann-Bell, 1998, p. 99.

The orbit of a comet can be defined with six numbers, called the orbital elements, and by entering these numbers into your favorite planetarium software, you can view the location of the comet at any given time reasonably near the epoch date. The epoch date is a particular date for which the orbital elements are calculated and therefore the most accurate around that time.

Different sets of six parameters can be used, but the most common are shown below. Example values are given for Comet Holmes (17P), which exhibited a remarkable outburst in October 2007, now almost 12 years ago.

Perihelion distance, q

This is the center-to-center distance from the comet to the Sun when the comet is at perihelion, its closest point to the Sun. For Comet Holmes, this is 2.05338 AU, well beyond the orbits of both the Earth and Mars.

Orbital eccentricity, e

This is a unitless number that is the measure of the amount of ellipticity an orbit has. For a circular orbit, e = 0. A parabolic orbit, e = 1. A hyperbolic orbit, e > 1. Many comets have highly elliptical orbits, often with e > 0.9. Short-period comets, such as Comet Holmes (17P), have more modest eccentricities. Comet Holmes has an orbital eccentricity of 0.432876. This means that at perihelion, Comet Holmes is 43.3% closer to the Sun than its midpoint distance, and at aphelion Comet Holmes is 43.3% further away from the Sun than its midpoint distance.

Date of perihelion, T

This is a date (converted to decimal Julian date) that the comet reached perihelion, or will next reach perihelion. For example, Comet Holmes reached perihelion on 2007 May 5.0284.

Inclination to the Ecliptic Plane, i

This is the angle made by the intersection of the plane of the comet’s orbit with the ecliptic, the plane of the Earth’s orbit. Comet Holmes has an inclination angle of 19.1143°.

Longitude of the ascending node, Ω

The intersection between the comet’s orbital plane and the Earth’s orbital plane forms a line, called the line of nodes. The places where this line intersects the comet’s orbit forms two points. One point defines the location where the comet crosses the ecliptic plane heading from south to north. This is called the ascending node. The other point defines the location where the comet crosses the ecliptic plane heading from north to south. This is called the descending node. 0° longitude is arbitrarily defined to be the direction of the vernal equinox, the point in the sky where the Sun in its apparent path relative to the background stars crosses the celestial equator heading north. The longitude of the ascending node (capital Omega, Ω) is the angle, measured eastward (in the direction of the Earth’s orbital motion) from the vernal equinox to the ascending node of the comet’s orbit. For Comet Holmes, that angle is 326.8532°.

Argument of perihelion, ω

The angle along the comet’s orbit in the direction of the comet’s motion between its perihelion point and its ascending node (relative to the ecliptic plane) is called the argument of perihelion (small omega, ω). For Comet Holmes, this angle is 24.352°.

If all the mass of the Sun and the comet were concentrated at a geometric point, and if they were the only two objects in the universe, these six orbital elements would be fixed for all time. But these two objects have physical size, and are affected by the gravitational pull of other objects in our solar system and beyond. Moreover, nongravitational forces can act on the comet’s nucleus, such as jets of material spewing out into space, exerting a tiny but non-negligible thrust on the comet, thus altering its orbit. Because of these effects, in practice it is a good idea to define a set of osculating orbital elements which will give the best positions for the comet around a particular date. These osculating orbital elements change gradually with time (due to gravitational perturbations and non-gravitational forces acting on the comet) and give the best approximation to the orbit at a given point in time. The further one strays from the epoch date for the osculating elements, the less accurate the predicted position of the comet will be.

For example, the IAU Minor Planet Center gives a set of orbital elements for Comet Holmes that has a more recent epoch date than the one given by the JPL Small-Body Database Browser. The MPC gives an epoch date of 2015 Jun 27.0, reasonably near the date of the most recent perihelion passage of this P = 6.89^y comet (2014 Mar 27.5736). JPL, on the other hand, provides a default epoch date of 2010 Jan 17.0, nearer the date of the 2007 May 5.0284 perihelion and the spectacular October 2007 apparition. For the most accurate current position of Comet Holmes in your planetarium software, you’ll probably want to use the MPC orbital elements, since they are for an epoch nearest to the date when you’ll be making your observations.

In this year of 2019, the best dates and times for observing the zodiacal light are listed below. The sky must be very clear with little or no light pollution. The specific times listed are for Dodgeville, Wisconsin.

2019	Begin	End	Direction
Tue. Jan. 22	6:39 p.m.	7:03 p.m.	West
Wed. Jan. 23	6:40 p.m.	7:40 p.m.	West
Thu. Jan. 24	6:41 p.m.	7:41 p.m.	West
Fri. Jan. 25	6:42 p.m.	7:42 p.m.	West
Sat. Jan. 26	6:43 p.m.	7:43 p.m.	West
Sun. Jan. 27	6:44 p.m.	7:44 p.m.	West
Mon. Jan. 28	6:45 p.m.	7:45 p.m.	West
Tue. Jan. 29	6:46 p.m.	7:46 p.m.	West
Wed. Jan. 30	6:48 p.m.	7:48 p.m.	West
Thu. Jan. 31	6:49 p.m.	7:49 p.m.	West
Fri. Feb. 1	6:50 p.m.	7:50 p.m.	West
Sat. Feb. 2	6:51 p.m.	7:51 p.m.	West
Sun. Feb. 3	6:52 p.m.	7:52 p.m.	West
Mon. Feb. 4	6:53 p.m.	7:53 p.m.	West
Tue. Feb. 5	6:55 p.m.	7:55 p.m.	West
Wed. Feb. 6	7:09 p.m.	7:56 p.m.	West

Thu. Feb. 21	7:14 p.m.	8:14 p.m.	West
Fri. Feb. 22	7:15 p.m.	8:15 p.m.	West
Sat. Feb. 23	7:16 p.m.	8:16 p.m.	West
Sun. Feb. 24	7:17 p.m.	8:17 p.m.	West
Mon. Feb. 25	7:19 p.m.	8:19 p.m.	West
Tue. Feb. 26	7:20 p.m.	8:20 p.m.	West
Wed. Feb. 27	7:21 p.m.	8:21 p.m.	West
Thu. Feb. 28	7:22 p.m.	8:22 p.m.	West
Fri. Mar. 1	7:23 p.m.	8:23 p.m.	West
Sat. Mar. 2	7:25 p.m.	8:25 p.m.	West
Sun. Mar. 3	7:26 p.m.	8:26 p.m.	West
Mon. Mar. 4	7:27 p.m.	8:27 p.m.	West
Tue. Mar. 5	7:28 p.m.	8:28 p.m.	West
Wed. Mar. 6	7:30 p.m.	8:30 p.m.	West
Thu. Mar. 7	7:31 p.m.	8:31 p.m.	West
Fri. Mar. 8	8:01 p.m.	8:32 p.m.	West

Fri. Mar. 22	8:50 p.m.	9:24 p.m.	West
Sat. Mar. 23	8:52 p.m.	9:52 p.m.	West
Sun. Mar. 24	8:53 p.m.	9:53 p.m.	West
Mon. Mar. 25	8:54 p.m.	9:54 p.m.	West
Tue. Mar. 26	8:56 p.m.	9:56 p.m.	West
Wed. Mar. 27	8:57 p.m.	9:57 p.m.	West
Thu. Mar. 28	8:59 p.m.	9:59 p.m.	West
Fri. Mar. 29	9:00 p.m.	10:00 p.m.	West
Sat. Mar. 30	9:01 p.m.	10:01 p.m.	West
Sun. Mar. 31	9:03 p.m.	10:03 p.m.	West
Mon. Apr. 1	9:04 p.m.	10:04 p.m.	West
Tue. Apr. 2	9:06 p.m.	10:06 p.m.	West
Wed. Apr. 3	9:07 p.m.	10:07 p.m.	West
Thu. Apr. 4	9:09 p.m.	10:09 p.m.	West
Fri. Apr. 5	9:10 p.m.	10:10 p.m.	West
Sat. Apr. 6	9:12 p.m.	10:12 p.m.	West
Sun. Apr. 7	10:03 p.m.	10:13 p.m.	West

Thu. Aug. 29	3:39 a.m.	4:39 a.m.	East
Fri. Aug. 30	3:40 a.m.	4:40 a.m.	East
Sat. Aug. 31	3:42 a.m.	4:42 a.m.	East
Sun. Sep. 1	3:43 a.m.	4:43 a.m.	East
Mon. Sep. 2	3:45 a.m.	4:45 a.m.	East
Tue. Sep. 3	3:46 a.m.	4:46 a.m.	East
Wed. Sep. 4	3:48 a.m.	4:48 a.m.	East
Thu. Sep. 5	3:49 a.m.	4:49 a.m.	East
Fri. Sep. 6	3:50 a.m.	4:50 a.m.	East
Sat. Sep. 7	3:52 a.m.	4:52 a.m.	East
Sun. Sep. 8	3:53 a.m.	4:53 a.m.	East
Mon. Sep. 9	3:55 a.m.	4:55 a.m.	East
Tue. Sep. 10	3:56 a.m.	4:56 a.m.	East
Wed. Sep. 11	3:57 a.m.	4:57 a.m.	East
Thu. Sep. 12	4:52 a.m.	4:59 a.m.	East

Fri. Sep. 27	5:11 a.m.	5:18 a.m.	East
Sat. Sep. 28	4:19 a.m.	5:19 a.m.	East
Sun. Sep. 29	4:20 a.m.	5:20 a.m.	East
Mon. Sep. 30	4:21 a.m.	5:21 a.m.	East
Tue. Oct. 1	4:23 a.m.	5:23 a.m.	East
Wed. Oct. 2	4:24 a.m.	5:24 a.m.	East
Thu. Oct. 3	4:25 a.m.	5:25 a.m.	East
Fri. Oct. 4	4:26 a.m.	5:26 a.m.	East
Sat. Oct. 5	4:27 a.m.	5:27 a.m.	East
Sun. Oct. 6	4:29 a.m.	5:29 a.m.	East
Mon. Oct. 7	4:30 a.m.	5:30 a.m.	East
Tue. Oct. 8	4:31 a.m.	5:31 a.m.	East
Wed. Oct. 9	4:32 a.m.	5:32 a.m.	East
Thu. Oct. 10	4:33 a.m.	5:33 a.m.	East
Fri. Oct. 11	4:43 a.m.	5:34 a.m.	East

Sat. Oct. 26	4:51 a.m.	5:19 a.m.	East
Sun. Oct. 27	4:53 a.m.	5:53 a.m.	East
Mon. Oct. 28	4:54 a.m.	5:54 a.m.	East
Tue. Oct. 29	4:55 a.m.	5:55 a.m.	East
Wed. Oct. 30	4:56 a.m.	5:56 a.m.	East
Thu. Oct. 31	4:57 a.m.	5:57 a.m.	East
Fri. Nov. 1	4:58 a.m.	5:58 a.m.	East
Sat. Nov. 2	4:59 a.m.	5:59 a.m.	East
Sun. Nov. 3	4:01 a.m.	5:01 a.m.	East
Mon. Nov. 4	4:02 a.m.	5:02 a.m.	East
Tue. Nov. 5	4:03 a.m.	5:03 a.m.	East
Wed. Nov. 6	4:04 a.m.	5:04 a.m.	East
Thu. Nov. 7	4:05 a.m.	5:05 a.m.	East
Fri. Nov. 8	4:06 a.m.	5:06 a.m.	East
Sat. Nov. 9	4:07 a.m.	5:07 a.m.	East
Sun. Nov. 10	4:34 a.m.	5:08 a.m.	East

Sun. Nov. 24	4:23 a.m.	4:27 a.m.	East
Mon. Nov. 25	4:24 a.m.	5:24 a.m.	East
Tue. Nov. 26	4:25 a.m.	5:25 a.m.	East
Wed. Nov. 27	4:26 a.m.	5:26 a.m.	East
Thu. Nov. 28	4:27 a.m.	5:27 a.m.	East
Fri. Nov. 29	4:28 a.m.	5:28 a.m.	East
Sat. Nov. 30	4:29 a.m.	5:29 a.m.	East
Sun. Dec. 1	4:30 a.m.	5:30 a.m.	East
Mon. Dec. 2	4:31 a.m.	5:31 a.m.	East
Tue. Dec. 3	4:32 a.m.	5:32 a.m.	East
Wed. Dec. 4	4:33 a.m.	5:33 a.m.	East
Thu. Dec. 5	4:34 a.m.	5:34 a.m.	East
Fri. Dec. 6	4:35 a.m.	5:35 a.m.	East
Sat. Dec. 7	4:35 a.m.	5:35 a.m.	East
Sun. Dec. 8	4:36 a.m.	5:36 a.m.	East
Mon. Dec. 9	4:37 a.m.	5:37 a.m.	East
Tue. Dec. 10	5:29 a.m.	5:38 a.m.	East

If you are interested in calculating the angle the ecliptic makes with your horizon for any date and time, you can use the following formula:

$\cos I = \cos \varepsilon \sin \phi-\sin \varepsilon \cos \phi \sin \theta$

where I is the angle between the ecliptic and the horizon, ε is  the obliquity of the ecliptic, φ is the latitude of the observer, and θ is the local sidereal time (the right ascension of objects on the observer's meridian at the time of observation).

Here’s a SAS program I wrote to do these calculations:

Zodiacal Light.sas Download

References
Meeus, J. Astronomical Algorithms. 2nd ed., Willmann-Bell, 1998, p. 99.

The Earth orbits the Sun once every 365.256363 (mean solar) days relative to the distant stars. The Earth’s orbital speed ranges from 18.2 miles per second at aphelion, around July 4th, to 18.8 miles per second at perihelion, around January 3rd. In units we’re perhaps more familiar with, that’s 65,518 mph at aphelion and 67,741 mph at perihelion. That’s a difference of 2,223 miles per hour!

As we are on a spinning globe, the direction towards which the Earth is orbiting is different at different times of the day. When the Sun crosses the celestial meridian, due south, at its highest point in the sky around noon (1:00 p.m. daylight time), the Earth is orbiting towards your right (west) as you are facing south. Since the Earth is orbiting towards the west, the Sun appears to move towards the east, relative to the background stars—if we could see them during the day. Since there are 360° in a circle and the Earth orbits the Sun in 365.256363 days (therefore the Sun appears to go around the Earth once every 365.256363 days relative to the background stars), the Sun’s average angular velocity eastward relative to the background stars is 360°/365.256363 days = 0.9856° per day.

The constellations through which the Sun moves are called the zodiacal constellations, and historically the zodiac contained 12 constellations, the same number as the number of months in a year. But Belgian astronomer Eugène Delporte (1882-1955) drew up the 88 constellation boundaries we use today, approved by the IAU in 1930, so now the Sun spends a few days each year in the non-zodiacal constellation Ophiuchus, the Serpent Bearer. Furthermore, because the Earth’s axis is precessing, the calendar dates during which the Sun is in a particular zodiacal constellation is gradually getting later.

Astrologically, each zodiacal constellation has a width of 30° (360° / 12 constellations = 30° per constellation). But, of course, the constellations are different sizes and shapes, so astronomically the number of days the Sun spends in each constellation varies. Here is the situation at present.

Constellation

Description

Sun Travel Dates

Capricornus

Sea Goat

Jan 19 through Feb 16

Aquarius

Water Bearer

Feb 16 through Mar 12

Pisces

The Fish

Mar 12 through Apr 18

Aries

The Ram

Apr 18 through May 14

Taurus

The Bull

May 14 through Jun 21

Gemini

The Twins

Jun 21 through Jul 20

Cancer

The Crab

Jul 20 through Aug 10

Leo

The Lion

Aug 10 through Sep 16

Virgo

The Virgin

Sep 16 through Oct 31

Libra

The Scales

Oct 31 through Nov 23

Scorpius

The Scorpion

Nov 23 through Nov 29

Ophiuchus

Serpent Bearer

Nov 29 through Dec 18

Sagittarius

The Archer

Dec 18 through Jan 19

The apparent path the Sun takes across the sky relative to the background stars through these 13 constellations is called the ecliptic. A little contemplation, aided perhaps by a drawing, will convince you that the ecliptic is also the plane of the Earth’s orbit around the Sun. The Moon never strays very far from the ecliptic in our sky, since its orbital plane around the Earth is inclined at a modest angle of 5.16° relative to the Earth’s orbital plane around the Sun. But, relative to the Earth’s equatorial plane, the inclination of the Moon’s orbit varies between 18.28° and 28.60° over 18.6 years as the line of intersection between the Moon’s orbital plane and the ecliptic plane precesses westward along the ecliptic due to the gravitational tug of war the Earth and the Sun exert on the Moon as it moves through space. This steep inclination to the equatorial plane is very unusual for such a large moon. In fact, all four satellites in our solar system that are larger than our Moon (Ganymede, Titan, Callisto, and Io) and the one that is slightly smaller (Europa) all orbit in a plane that is inclined less than 1/2° from the equatorial plane of their host planet (Jupiter and Saturn).

Since the Moon is never farther than 5.16° from the ecliptic, its apparent motion through our sky as it orbits the Earth mimics that of the Sun, only the Moon’s angular speed is over 13 times faster, completing its circuit of the sky every 27.321662 days, relative to the distant stars. Thus the Moon moves a little over 13° eastward every day, or about 1/2° per hour. Since the angular diameter of the Moon is also about 1/2°, we can easily remember that the Moon moves its own diameter eastward relative to the stars every hour. Of course, superimposed on this motion is the 27-times-faster-yet motion of the Moon and stars westward as the Earth rotates towards the east.

Now, take a look at the following table and see how the Moon’s motion mimics that of the Sun throughout the month, and throughout the year.

——— Moon’s Phase and Path ———

Date

Sun’s Path

New

Full

Mar 20

EQ ↑

High

EQ ↓

Low

Jun 21

High

EQ ↓

Low

EQ ↑

Sep 22

EQ ↓

Low

EQ ↑

High

Dec 21

Low

EQ ↑

High

EQ ↓

New = New Moon

near the Sun

FQ = First Quarter

90° east of the Sun

Full = Full Moon

180°, opposite the Sun

LQ = Last Quarter

90° west of the Sun

EQ ↑

= crosses the celestial equator heading north

High

= rides high (north) across the sky

EQ ↓

= crosses the celestial equator heading south

Low

= rides low (south) across the sky

So, if you aren’t already doing so, take note of how the Moon moves across the sky at different phases and times of the year. For example, notice how the full moon (nearest the summer solstice) on June 27/28 rides low in the south across the sky. You’ll note the entry for the “Jun 21” row and “Full” column is “Low”. And, the Sun entry for that date is “High”. See, it works!

1
Zodiacal Light 9:05 – 10:05 p.m. West

2
Zodiacal Light 9:11 – 10:06 p.m. West

Over the eons, as comets shed dust and asteroids collide, dust particles are freed from their parent bodies and, for a time, orbit independently around the Sun. These tiny particles (typically 1 to 300 μm across) reflect sunlight that can be seen from Earth. This phenomenon is called the zodiacal light (pronounced zoe-DYE-uh-cul). It is a subtle yet beautiful cone of white light most easily seen extending up from the western horizon at the end of evening twilight, or projecting above the eastern horizon just before morning twilight begins. This phenomenon is named after the zodiac because the dust is concentrated near the plane of the ecliptic. The picture is complicated by the fact that there are zodiacal dust components that lie along the solar equatorial plane, the orbital plane of Venus, the invariable plane of the solar system, and the ecliptic. All four of these reference planes lie within a few degrees inclination of each other.

Since the zodiacal light is generally brightest along the ecliptic just a few degrees away from the Sun, it is best to pick a time of year when that portion of the ecliptic is most nearly perpendicular to the horizon to make your observations. This, of course, depends on your latitude (closer to the equator being better), but for those of us here in the Midwest, February, March, and April offer the very best times to see and photograph the zodiacal light above the western horizon at the end of evening twilight. The very best times to see and photograph the zodiacal light above the eastern horizon before the beginning of morning twilight occurs for us in August, September and October.

In the images below, the yellow line is the ecliptic. A mid-month view for each month of the year, morning and evening, is shown for latitude 43° N. Note that the best months for viewing evening and morning zodiacal light listed above show the ecliptic at the steepest angles relative to the horizon.

In this year of 2017, the best dates and times for observing the zodiacal light are listed below. The sky must be very clear. The specific times listed are for Dodgeville, Wisconsin.

2017	Begin	End	Direction
Sun. Feb. 12	7:03 p.m.	7:32 p.m.	West
Mon. Feb. 13	7:05 p.m.	8:05 p.m.	West
Tue. Feb. 14	7:06 p.m.	8:06 p.m.	West
Wed. Feb. 15	7:07 p.m.	8:07 p.m.	West
Thu. Feb. 16	7:08 p.m.	8:08 p.m.	West
Fri. Feb. 17	7:09 p.m.	8:09 p.m.	West
Sat. Feb. 18	7:11 p.m.	8:11 p.m.	West
Sun. Feb. 19	7:12 p.m.	8:12 p.m.	West
Mon. Feb. 20	7:13 p.m.	8:13 p.m.	West
Tue. Feb. 21	7:14 p.m.	8:14 p.m.	West
Wed. Feb. 22	7:15 p.m.	8:15 p.m.	West
Thu. Feb. 23	7:17 p.m.	8:17 p.m.	West
Fri. Feb. 24	7:18 p.m.	8:18 p.m.	West
Sat. Feb. 25	7:19 p.m.	8:19 p.m.	West
Sun. Feb. 26	7:20 p.m.	8:20 p.m.	West
Mon. Feb. 27	7:22 p.m.	8:22 p.m.	West

Tue. Mar. 14	8:40 p.m.	9:22 p.m.	West
Wed. Mar. 15	8:42 p.m.	9:42 p.m.	West
Thu. Mar. 16	8:43 p.m.	9:43 p.m.	West
Fri. Mar. 17	8:44 p.m.	9:44 p.m.	West
Sat. Mar. 18	8:46 p.m.	9:46 p.m.	West
Sun. Mar. 19	8:47 p.m.	9:47 p.m.	West
Mon. Mar. 20	8:48 p.m.	9:48 p.m.	West
Tue. Mar. 21	8:50 p.m.	9:50 p.m.	West
Wed. Mar. 22	8:51 p.m.	9:51 p.m.	West
Thu. Mar. 23	8:52 p.m.	9:52 p.m.	West
Fri. Mar. 24	8:54 p.m.	9:54 p.m.	West
Sat. Mar. 25	8:55 p.m.	9:55 p.m.	West
Sun. Mar. 26	8:56 p.m.	9:56 p.m.	West
Mon. Mar. 27	8:58 p.m.	9:58 p.m.	West
Tue. Mar. 28	8:59 p.m.	9:59 p.m.	West
Wed. Mar. 29	9:27 p.m.	10:01 p.m.	West

Thu. Apr. 13	9:23 p.m.	10:07 p.m.	West
Fri. Apr. 14	9:25 p.m.	10:25 p.m.	West
Sat. Apr. 15	9:26 p.m.	10:26 p.m.	West
Sun. Apr. 16	9:28 p.m.	10:28 p.m.	West
Mon. Apr. 17	9:29 p.m.	10:29 p.m.	West
Tue. Apr. 18	9:31 p.m.	10:31 p.m.	West
Wed. Apr. 19	9:33 p.m.	10:33 p.m.	West
Thu. Apr. 20	9:34 p.m.	10:34 p.m.	West
Fri. Apr. 21	9:36 p.m.	10:36 p.m.	West
Sat. Apr. 22	9:38 p.m.	10:38 p.m.	West
Sun. Apr. 23	9:39 p.m.	10:39 p.m.	West
Mon. Apr. 24	9:41 p.m.	10:41 p.m.	West
Tue. Apr. 25	9:43 p.m.	10:43 p.m.	West
Wed. Apr. 26	9:44 p.m.	10:44 p.m.	West
Thu. Apr. 27	9:46 p.m.	10:46 p.m.	West

Sat. Aug. 19	3:24 a.m.	3:40 a.m.	East
Sun. Aug. 20	3:26 a.m.	4:26 a.m.	East
Mon. Aug. 21	3:27 a.m.	4:27 a.m.	East
Tue. Aug. 22	3:29 a.m.	4:29 a.m.	East
Wed. Aug. 23	3:30 a.m.	4:30 a.m.	East
Thu. Aug. 24	3:32 a.m.	4:32 a.m.	East
Fri. Aug. 25	3:33 a.m.	4:33 a.m.	East
Sat. Aug. 26	3:35 a.m.	4:35 a.m.	East
Sun. Aug. 27	3:26 a.m.	4:36 a.m.	East
Mon. Aug. 28	3:38 a.m.	4:38 a.m.	East
Tue. Aug. 29	3:39 a.m.	4:39 a.m.	East
Wed. Aug. 30	3:41 a.m.	4:41 a.m.	East
Thu. Aug. 31	3:42 a.m.	4:42 a.m.	East
Fri. Sep. 1	3:44 a.m.	4:44 a.m.	East
Sat. Sep. 2	3:45 a.m.	4:45 a.m.	East
Sun. Sep. 3	3:47 a.m.	4:47 a.m.	East
Mon. Sep. 4	4:36 a.m.	4:48 a.m.	East

Mon. Sep. 18	4:07 a.m.	4:49 a.m.	East
Tue. Sep. 19	4:08 a.m.	5:08 a.m.	East
Wed. Sep. 20	4:10 a.m.	5:10 a.m.	East
Thu. Sep. 21	4:11 a.m.	5:11 a.m.	East
Fri. Sep. 22	4:12 a.m.	5:12 a.m.	East
Sat. Sep. 23	4:13 a.m.	5:13 a.m.	East
Sun. Sep. 24	4:15 a.m.	5:15 a.m.	East
Mon. Sep. 25	4:16 a.m.	5:16 a.m.	East
Tue. Sep. 26	4:17 a.m.	5:17 a.m.	East
Wed. Sep. 27	4:18 a.m.	5:18 a.m.	East
Thu. Sep. 28	4:20 a.m.	5:20 a.m.	East
Fri. Sep. 29	4:21 a.m.	5:21 a.m.	East
Sat. Sep. 30	4:22 a.m.	5:22 a.m.	East
Sun. Oct. 1	4:23 a.m.	5:23 a.m.	East
Mon. Oct. 2	4:24 a.m.	5:24 a.m.	East
Tue. Oct. 3	4:26 a.m.	5:26 a.m.	East

Wed. Oct. 18	4:43 a.m.	5:43 a.m.	East
Thu. Oct. 19	4:44 a.m.	5:44 a.m.	East
Fri. Oct. 20	4:45 a.m.	5:45 a.m.	East
Sat. Oct. 21	4:46 a.m.	5:46 a.m.	East
Sun. Oct. 22	4:48 a.m.	5:48 a.m.	East
Mon. Oct. 23	4:49 a.m.	5:49 a.m.	East
Tue. Oct. 24	4:50 a.m.	5:50 a.m.	East
Wed. Oct. 25	4:51 a.m.	5:51 a.m.	East
Thu. Oct. 26	4:52 a.m.	5:52 a.m.	East
Fri. Oct. 27	4:53 a.m.	5:53 a.m.	East
Sat. Oct. 28	4:54 a.m.	5:54 a.m.	East
Sun. Oct. 29	4:55 a.m.	5:55 a.m.	East
Mon. Oct. 30	4:57 a.m.	5:57 a.m.	East
Tue. Oct. 31	4:58 a.m.	5:58 a.m.	East
Wed. Nov. 1	4:59 a.m.	5:59 a.m.	East
Thu. Nov. 2	5:27 a.m.	6:00 a.m.	East

On the February, March, and April evenings listed above, you will see a broad, faint band of light extending upwards from the western horizon, sloping a little to the left, and reaching nearly halfway to the top of the sky.

On the August, September, and October mornings listed above, you will see a broad, faint band of light extending upwards from the eastern horizon, sloping a little to the right, and reaching nearly halfway to the top of the sky.

It is essential that your view is not spoiled by nearby lights or any city to the west (Feb-Apr) or east (Aug-Oct). Give your eyes a few minutes to adjust to the darkness. Slowly sweeping your eyes back and forth from southwest to northwest (Feb-Apr) or northeast to southeast (Aug-Oct) will help you spot the zodiacal light band. Once spotted, you should be able to see it without moving your head.

On the February, March, and April evenings listed above, the zodiacal light is best seen right at the end of evening twilight, and remains visible for an hour or so after that.

On the August, September, and October mornings listed above, the zodiacal light is best seen about an hour or so before the beginning of morning twilight, right up to the beginning of morning twilight.

Enjoy!