Like Sun, Like Moon

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
FQ
Full
LQ
Mar 20
EQ
EQ
High
EQ
Low
Jun 21
High
High
EQ
Low
EQ
Sep 22
EQ
EQ
Low
EQ
High
Dec 21
Low
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!

The Zodiacal Light

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!