## Comet ATLAS (C/2019 Y4)

Comet C/2019 Y4 ATLAS was discovered on December 28, 2019 and is named after the observational program that discovered it: Asteroid Terrestrial-impact Last Alert System (ATLAS). It could become a naked-eye comet—if it doesn’t disintegrate as it gets closer to the Sun. Here’s an ephemeris for the remainder of April and May.

## Shadows Cast by Starlight

Henry Norris Russell (1877-1957) received his Ph.D. at Princeton in 1899 at just 21 years of age. Three years later—in 1902 when he was 24 years old and years before his discovery of the color-luminosity relationship now known as the Hertzsprung-Russell (H-R) diagram—Russell had an interesting article published in the journal Popular Astronomy that shows him already to be a meticulous and perspicacious observational astronomer. This article, completed 118 years ago this day, is reprinted below.

SHADOWS CAST BY STARLIGHT.

HENRY NORRIS RUSSELL.

FOR POPULAR ASTRONOMY.

It has long been known that Venus casts a distinct shadow; and the same thing has sometimes been observed in Jupiter’s case. More recently, it has been stated in the daily press* that shadows cast by Sirius have been seen at the Harvard Observatory in Jamaica, though it was then said that they could probably be seen only where the air is exceptionally clear.

The writer began to investigate this subject, quite independently, last November, and has found that the shadows cast by a number of the brighter fixed stars can be seen without difficulty under ordinary circumstances, provided proper precautions are taken to exclude extraneous light, and to secure the maximum sensitiveness of the observer’s eyes.

* Interview with Professor W.H. Pickering, New York Tribune, Jan. 18, 1902.

The most convenient method of observation is as follows: Choose a window from which the star is visible, while as little light as possible enters from terrestrial sources. Darken the room completely, with the exception of this window. Open the window, and screen down its aperture to an area of a square foot or less. Hold a large piece of white paper in the path of the star’s rays, as far from the opening as possible. The image of the opening will then appear on the paper.

It cannot, however, be well seen until the observer has spent at least ten minutes in the dark, (to rest his eyes from the glare of ordinary lights). The paper should be held within a foot or so of the eyes, as the faint patch of starlight is most easily visible when its apparent area is large. The shadow of any convenient object may now be made to fall on the screen, and may be observed. By holding the object near the window and noticing that its shadow is still sharp, the observer may convince himself that the light which casts the shadow really comes from the star.

By the method above described, the writer has succeeded in distinguishing shadows cast by the following stars, (which are here arranged in order of brightness):

 Mag. Mag. α Canis Majoris (Sirius) – 1.4 ζ Orionis 1.9 α Bootis (Arcturus) 0.0 β Tauri 1.9 α Aurigae (Capella) 0.2 γ Geminorum 2.0 β Orionis (Rigel) 0.3 β Canis Majoris 2.0 α Canis Minoris (Procyon) 0.5 α Hydrae 2.0 α Orionis* (Betelgeuse) 0.8? α Arietis 2.0 α Tauri (Aldebaran) 1.0 κ Orionis 2.2 β Geminorum (Pollux) 1.1 β Leonis 2.2 α Virginis (Spica) 1.2 γ Leonis 2.2 α Leonis (Regulus) 1.4 δ Orionis 2.4 ε Canis Majoris 1.5 η Canis Majoris 2.4 α Geminorum (Castor) 1.6 ζ Argus 2.5 ε Orionis 1.8 α Ceti 2.7 δ Canis Majoris 1.9 15 Argus 2.9 γ Orionis 1.9

* Variable

The groups of stars comprised in the Pleiades and the sword of Orion also cast perceptible shadows. With a wide open window the belt of Orion should be added to this class.

Most of the observations on which this list is based were made at Princeton on February 7th, and 8th, and March 6th, 1902. The first of these nights is recorded as not remarkably clear, the others as very clear. Whenever there was any doubt of the reality of an observed patch of starlight, it was located at least three times, and it was verified each time that the star was really visible from the spot where its light had been located. Many more stars might have been added to the 29 in the foregoing list, had not unfriendly street lamps confined the observations to less than half the sky.

As many of the stars observed were at a low altitude, it may be concluded that a star of the 3rd magnitude, if near the zenith, would cast a perceptible shadow.

In attempting to get a shadow from these faint stars, the opening of the window should be narrowed to a width of a few inches, so as to cut off as much as possible of the diffused light of the sky. Care should be taken not to look at the sky while observing, as it is bright enough to dazzle the eyes for some little time.

By observing these precautions, the writer has been able to detect shadows cast by Sirius, Arcturus and Capella on moonlight nights,—in the case of Sirius, even when the Moon shone into the room.

The actual brightness of the screen, even when illuminated by Sirius, is very small in comparison with that of the “dark” background of the sky, as seen by the naked eye. White paper reflects about 80 per cent of the incident light. From photometric considerations, a disk of this material 1° in apparent diameter, illuminated perpendicularly by Sirius, should send us about 1/16,000 as much light as the star.

But, according to Professor Newcomb’s determination*, an area of sky 1° in diameter, remote from the Milky Way, sends us 9/10 as much light as a 5th magnitude star, or about 1/400 of the light of Sirius. Hence the sky is about 40 times as bright, area for area, as the paper illuminated by Sirius. The illumination of the paper by a 1st magnitude star is about 1/400 as bright, and by a 3d magnitude star less than 1/2000 as bright, area for area, as the “dark” background of the sky.

* Astrophysical Journal, December 1901.

This faint light, as might be anticipated, shows no perceptible color. The light of the white stars β and γ Orionis and the red star α Orionis does not differ sensibly in quality; but the light of the red star appears much fainter than the star’s brightness, as directly seen, would lead one to anticipate. On the screen, the light of α Orionis is much fainter than that of β, and only a little brighter than that of γ, while by direct vision α is much nearer to β than to γ in brightness. As β is 1 ½ magnitudes brighter than γ, it appears that, as measured by the intensity of its light on a screen, α Orionis is at least half a magnitude, perhaps a whole magnitude, fainter than when compared with the neighboring white stars by direct vision.

Such a result might have been anticipated à priori, since, in the ease of such faint lights as are here dealt with, the eye is sensitive to the green part of the spectrum alone, and this is relatively brighter in the spectrum of a white star than of a red one.

A much more interesting example of the accordance of theoretical prediction with observation is afforded by another phenomenon discovered by the writer, which is not hard to observe.

A surface illuminated by a planet—Venus for example—appears uniformly and evenly bright, but in the case of a fixed star, there are marked variations in brightness, so that the screen appears covered with moving dark markings.

This was predicted many years ago by Professor Young, in discussing the twinkling of the stars. He says*: “If the light of a star were strong enough, a white surface illuminated by it would look like the sandy bottom of a shallow, rippling pool of water illuminated by sunlight, with light and dark mottlings which move with the ripples on the surface. So, as we look toward the star, and the mottlings due to the irregularities of the air move by us, we see the star alternately bright and faint; in other words, it twinkles.”

General Astronomy, page 538 (edition of 1898).

It would be difficult to give a better description of the observed phenomenon than the one contained in the first part of the above quotation. It need only be added that the dark markings are much more conspicuous than the bright ones. This agrees with the fact that a star more frequently seems to lose light while twinkling than to gain it.

Sirius is the only star whose light is bright enough to make these light and dark mottlings visible without great difficulty, though the writer has seen them in the light of Rigel and Procyon. With Sirius they have been seen every time the star’s light has been observed on a moonless night. They are much more conspicuous when the star is twinkling violently than on nights when the air is steady. In the latter case there are only faint irregular mottlings, whose motion produces a flickering effect. More usually there appear also ill-defined dark bands, two or three inches wide. These are never quite straight nor parallel but usually show a preference for one or two directions, sometimes dividing the screen into irregular polygons. On some nights they merely seem to oscillate, but on others they have a progressive motion, which may be at any angle with their own direction. The rate of motion is very variable, but is greatest on windy nights,—another evidence of the atmospheric origin of the bands.

The best nights for observing these bands occur when the stars are twinkling strongly, and there is not much wind. The directions given above for observing shadows should be somewhat modified in this case.

If the room is not at the same temperature as the outer air, the window should be kept closed, as otherwise most of what is seen will be due to the air-currents near it. It is also desirable to have an area of star-light at least a foot square to see the bands in, so that a good sized part of the window should be left clear.

If Sirius is unavailable, Arcturus and Vega are probably the best stars in whose light to attempt to see the bands.

PRINCETON, N. J., March 24, 1902.

## Counting Stars

Looking in all directions, how many stars are there brighter than a particular visual magnitude? Here’s an empirical formula that gives an approximation. It can be used over the range mv = +4.0 to +25.0.

$\textup{S} = 10^{-0.0003\,\textup{m}^{3} + 0.0019\,\textup{m}^{2} + 0.484\,\textup{m} + 0.795}$

where S is the approximate number of stars brighter than apparent visual magnitude m in the entire sky

Apparent Visual Magnitude# of Stars
4.0552
4.1618
4.2690
4.3772
4.4863
4.5964
4.61,077
4.71,204
4.81,345
4.91,503
5.01,679
5.11,875
5.22,094
5.32,338
5.42,611
5.52,914
5.63,253
5.73,631
5.84,051
5.94,520
6.05,042
6.15,623
6.26,271
6.36,992
6.47,794
6.58,687
6.69,681
6.710,786
6.812,015
6.913,382
7.014,900
7.116,588
7.218,464
7.320,547
7.422,860
7.525,428
7.628,278
7.731,441
7.834,949
7.938,839
8.043,152
8.147,932
8.253,229
8.359,096
8.465,592
8.572,784
8.680,743
8.789,549
8.899,287
8.9110,055
9.0121,955
9.1135,104
9.2149,627
9.3165,662
9.4183,362
9.5202,891
9.6224,431
9.7248,181
9.8274,358
9.9303,200
10.0334,965
10.1369,938
10.2408,426
10.3450,768
10.4497,330
10.5548,514
10.6604,755
10.7666,528
10.8734,349
10.9808,780
11.0890,430
11.1979,963
11.21,078,096
11.31,185,610
11.41,303,349
11.51,432,229
11.61,573,241
11.71,727,456
11.81,896,035
11.92,080,230
12.02,281,392
12.12,500,983
12.22,740,574
12.33,001,863
12.43,286,675
12.53,596,976
12.63,934,877
12.74,302,651
12.84,702,734
12.95,137,742
13.05,610,480
13.16,123,951
13.26,681,371
13.37,286,180
13.47,942,053
13.58,652,916
13.69,422,957
13.710,256,640
13.811,158,721
13.912,134,260
14.013,188,640
14.114,327,575
14.215,557,134
14.316,883,749
14.418,314,236
14.519,855,805
14.621,516,082
14.723,303,122
14.825,225,420
14.927,291,933
15.029,512,092
15.131,895,815
15.234,453,520
15.337,196,142
15.440,135,142
15.543,282,516
15.646,650,811
15.750,253,128
15.854,103,131
15.958,215,053
16.062,603,700
16.167,284,449
16.272,273,253
16.377,586,632
16.483,241,673
16.589,256,016
16.695,647,847
16.7102,435,879
16.8109,639,337
16.9117,277,932
17.0125,371,840
17.1133,941,667
17.2143,008,417
17.3152,593,453
17.4162,718,451
17.5173,405,353
17.6184,676,315
17.7196,553,644
17.8209,059,737
17.9222,217,010
18.0236,047,823
18.1250,574,401
18.2265,818,743
18.3281,802,538
18.4298,547,061
18.5316,073,074
18.6334,400,717
18.7353,549,396
18.8373,537,665
18.9394,383,103
19.0416,102,189
19.1438,710,168
19.2462,220,923
19.3486,646,831
19.4511,998,631
19.5538,285,275
19.6565,513,790
19.7593,689,134
19.8622,814,048
19.9652,888,922
20.0683,911,647
20.1715,877,479
20.2748,778,904
20.3782,605,508
20.4817,343,852
20.5852,977,352
20.6889,486,170
20.7926,847,110
20.8965,033,523
20.91,004,015,228
21.01,043,758,439
21.11,084,225,707
21.21,125,375,873
21.31,167,164,044
21.41,209,541,573
21.51,252,456,065
21.61,295,851,393
21.71,339,667,742
21.81,383,841,658
21.91,428,306,130
22.01,472,990,684
22.11,517,821,499
22.21,562,721,546
22.31,607,610,744
22.41,652,406,140
22.51,697,022,107
22.61,741,370,568
22.71,785,361,232
22.81,828,901,853
22.91,871,898,516
23.01,914,255,925
23.11,955,877,722
23.21,996,666,815
23.32,036,525,723
23.42,075,356,932
23.52,113,063,265
23.62,149,548,260
23.72,184,716,557
23.82,218,474,290
23.92,250,729,483
24.02,281,392,450
24.12,310,376,189
24.22,337,596,778
24.32,362,973,766
24.42,386,430,550
24.52,407,894,751
24.62,427,298,570
24.72,444,579,131
24.82,459,678,812
24.92,472,545,544
25.02,483,133,105

How many stars are there in our Milky Way galaxy? Between 100 and 400 billion stars. Many stars are not very luminous, and can only be seen in the immediate solar neighborhood. That is one source of uncertainty.

How many galaxies are there in the observable universe? Something like two trillion (2 × 1012).

How many stars are in the observable universe? Something like a septillion (1024). A trillion trillion!

And, just so you know, our universe is probably much larger than the volume that we can observe.

How does the Universe love thee? Let us count the stars…

References

“How many stars are in the sky?”, Space Math, NASA Goddard Space Flight Center, accessed February 29, 2020, https://spacemath.gsfc.nasa.gov/weekly/6Page103.pdf.

Wikipedia contributors, “Galaxy,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Galaxy&oldid=942479372 (accessed February 29, 2020).

Wikipedia contributors, “Milky Way,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Milky_Way&oldid=942977760 (accessed February 29, 2020).

## Impetus for Iapetus

What a strange world Iapetus is! The third largest satellite of Saturn—and the outermost of Saturn’s large satellites—is a moon of many mysteries. We’ll take a look at three of them.

Mystery #1: Iapetus appears to be an original satellite of Saturn, and yet unlike the other regular satellites, its orbit is inclined 15.5˚ relative to Saturn’s equator. The reason for this steep inclination is not well understood.

And, oh, the view! Iapetus is the perfect perch to view Saturn’s rings, as it orbits Saturn every 79.3 days in its steeply inclined orbit.

Mystery #2: Iapetus has the largest albedo dichotomy in the solar system. Why? Iapetus is locked in synchronous rotation as it orbits around Saturn, with the leading hemisphere ten times darker than its trailing hemisphere.

It is thought that the natural state of the Iapetian surface is the bright icy part, with the dark material a thin veneer, less than a meter thick.

Mystery #3: Iapetus has a shape consistent with a body spinning every ~16 hours and yet its rotation period is 79.3 days, and it has a prominent ridge that can be followed 3/4 of the way around the equator.

The surface of Iapetus is heavily cratered, indicating it is very old. Could two comparable-sized objects have collided almost head-on billions of years ago to form Iapetus?

As beautiful as spacecraft flyby and orbital images are of Iapetus and the many other interesting moons in our solar system, can you imagine what vistas await us once we start exploring their surfaces with rovers? Anticipation of these images and scientific discoveries surely is an impetus to explore the surface of Iapetus (and other moons) sooner rather than later.

References

Bonnefoy, Léa E., Jean-François Lestrade, Emmanuel Lellouch, Alice Le Gall, Cédric Leyrat, Nicolas Ponthieu, and Bilal Ladjelate. “Probing the subsurface of the two faces of Iapetus.” arXiv preprint arXiv:1911.03394 (2019).

Leleu, Adrien, Martin Jutzi, and Martin Rubin. “The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets.” Nature astronomy 2, no. 7 (2018): 555-561.

Rivera-Valentin, Edgard G., Amy C. Barr, EJ Lopez Garcia, Michelle R. Kirchoff, and Paul M. Schenk. “Constraints on planetesimal disk mass from the cratering record and equatorial ridge on Iapetus.” The Astrophysical Journal 792, no. 2 (2014): 127.

## Satellite and Meteor Crossings 2019 #2

Edmund Weiss (1837-1917) and many astronomers since have called asteroids “vermin of the sky”, but on October 4, 1957 another “species” of sky vermin made its debut: artificial satellites.  In the process of video recording stars for possible asteroid occultations, I frequently see satellites passing through my 17 × 11 arcminute field of view.

I’ve put together a video montage of satellites I serendipitously recorded between August 9, 2019 and December 22, 2019.  Many of the satellite crossings are moving across the fields as “dashes” because of the longer integration times I need to use for some of my asteroid occultation work. A table of these events is shown below the video. The range is the distance between observer and satellite at the time of observation. North is up and east is to the left.

Satellites in higher orbits take longer to cross the field. In the next video, the originally geosynchronous satellite OPS 1570 (IMEWS-3, “Integrated Missile Early Warning System”) is barely visible until it exhibits an amazing sunglint around 3:41:22 UT.

I caught one meteor on October 6, 2019 at 9:57:43 UT. Field location was UCAC4 515-043597. The meteor was a Daytime Sextantid, as determined using the method I described previously in There’s a Meteor in My Image. The meteor even left a brief afterglow—a meteor train!

References
Hughes, D. W. & Marsden, B. G. 2007, J. Astron. Hist. Heritage, 10, 21

## An Astronomy Retirement Community

Are any of you nearing retirement (as I am) or already retired who might be interested in moving to an astronomy-oriented retirement community? If you are, I encourage you to join the moderated Groups.io discussion group Dark-Sky Communities at

https://groups.io/g/Dark-Sky-Communities

I am working to establish such a community and would value your input and assistance. That work involves extensive research, networking, writing articles in various publications to reach a wider audience, finding a suitable developer, and seeking benefactors.

Some characteristics of the community I envision include:

1. Rural location with a dark night sky, but not too far from a city with decent medical facilities, preferably to the northeast or northwest;
2. Location with an abundance of clear nights and mild winters, probably in Arizona, New Mexico, or West Texas;
3. Lighting within the community that does not interfere with astronomical activities, strictly enforced;
4. Community is owned and operated by a benefit corporation or cooperative that will rent a house or apartment to each resident;
5. Observatories will be available for rental by interested residents who will equip them;
6. Pro-am collaborative research opportunities will be developed and nurtured;
7. A community observatory and a public observatory for astronomy outreach will be constructed and maintained;
8. Lodging will be available for visitors and guests;
9. There will be opportunities for on-site income operating and maintaining the community or, alternatively, a reduction in monthly rental fees.

Many of us have spent a significant amount of time and energy over the years trying to rein in light pollution in our respective communities and in the wider world, with varying degrees of success. Those efforts should continue, but the grim reality is that light pollution is continuing to get worse almost everywhere.

The opportunity to live in a community of varied interests but with a common appreciation for the night sky and a natural nighttime environment will appeal to many of us. Furthermore, a dark-sky community will afford us opportunities to show the world at large a better way to live.

Traditionally, in the United States at least, if one wants to live under a dark and starry night sky, your only options are to purchase land and build a house on it, or purchase an existing rural home. Not only is buying and maintaining rural real estate unaffordable or impractical for many, many would prefer to live in a rural community, provided that the night sky and nighttime environment are vigorously protected. Rental will also make it easier to move into and out of the community as circumstances change.

## Zodiacal Light 2020

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:

 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

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:

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

## Meteor Shower Calendar 2020

Here’s our meteor shower calendar for 2020.  It is sourced from the IMO’s Working List of Visual Meteor Showers (https://www.imo.net/files/meteor-shower/cal2020.pdf, Table 5, p. 25).

Each meteor shower is identified using its three-character IAU meteor shower code.  Codes are bold on the date of maximum, and one day either side of maximum.

Here’s a printable PDF file of the meteor shower calendar shown below:

Happy meteor watching!

 January 2020
SUN MON TUE WED THU FRI SAT
1
DLM QUA
2
DLM QUA
3
DLM QUA
4
DLM QUA
5
DLM QUA
6
DLM QUA
7
DLM QUA
8
DLM QUA
9
DLM QUA
10
DLM QUA GUM
11
DLM QUA GUM
12
DLM QUA GUM
13
DLM GUM
14
DLM GUM
15
DLM GUM
16
DLM GUM
17
DLM GUM
18
DLM GUM
19
DLM GUM
20
DLM GUM
21
DLM GUM
22
DLM GUM
23
DLM
24
DLM
25
DLM
26
DLM
27
DLM
28
DLM
29
DLM
30
DLM
31
DLM ACE

 February 2020
SUN MON TUE WED THU FRI SAT
1
DLM ACE
2
DLM ACE
3
DLM ACE
4
DLM ACE
5
ACE
6
ACE
7
ACE
8
ACE
9
ACE
10
ACE
11
ACE
12
ACE
13
ACE
14
ACE
15
ACE
16
ACE
17
ACE
18
ACE
19
ACE
20
ACE
21 22
23 24 25
GNO
26
GNO
27
GNO
28
GNO

 March 2020
SUN MON TUE WED THU FRI SAT
1
GNO
2
GNO
3
GNO
4
GNO
5
GNO
6
GNO
7
GNO
8
GNO
9
GNO
10
GNO
11
GNO
12
GNO
13
GNO
14
GNO
15
GNO
16
GNO
17
GNO
18
GNO
19
GNO
20
GNO
21
GNO
22
GNO
23
GNO
24
GNO
25
GNO
26
GNO
27
GNO
28
GNO
29 30 31
 April 2020
SUN MON TUE WED THU FRI SAT
1 2 3 4
5 6 7 8 9 10 11
12 13 14
LYR
15
PPU LYR
16
PPU LYR
17
PPU LYR
18
PPU LYR
19
ETA PPU LYR
20
ETA PPU LYR
21
ETA PPU LYR
22
ETA PPU LYR
23
ETA PPU LYR
24
ETA PPU LYR
25
ETA PPU LYR
26
ETA PPU LYR
27
ETA PPU LYR
28
ETA PPU LYR
29
ETA LYR
30
ETA LYR

 May 2020
SUN MON TUE WED THU FRI SAT
1
ETA
2
ETA
3
ELY ETA
4
ELY ETA
5
ELY ETA
6
ELY ETA
7
ELY ETA
8
ELY ETA
9
ELY ETA
10
ELY ETA
11
ELY ETA
12
ELY ETA
13
ELY ETA
14
ARI ELY ETA
15
ARI ETA
16
ARI ETA
17
ARI ETA
18
ARI ETA
19
ARI ETA
20
ARI ETA
21
ARI ETA
22
ARI ETA
23
ARI ETA
24
ARI ETA
25
ARI ETA
26
ARI ETA
27
ARI ETA
28
ARI ETA
29
ARI
30
ARI
31
ARI

 June 2020
SUN MON TUE WED THU FRI SAT
1
ARI
2
ARI
3
ARI
4
ARI
5
ARI
6
ARI
7
ARI
8
ARI
9
ARI
10
ARI
11
ARI
12
ARI
13
ARI
14
ARI
15
ARI
16
ARI
17
ARI
18
ARI
19
ARI
20
ARI
21
ARI
22
JBO ARI
23
JBO ARI
24
JBO ARI
25
JBO
26
JBO
27
JBO
28
JBO
29
JBO
30
JBO

 July 2020
SUN MON TUE WED THU FRI SAT
1
JBO
2
JBO
3
CAP
4
CAP
5
CAP
6
CAP
7
CAP
8
CAP
9
CAP
10
CAP
11
CAP
12
CAP SDA
13
CAP SDA
14
CAP SDA
15
CAP SDA PAU
16
CAP SDA PAU
17
PER CAP SDA PAU
18
PER CAP SDA PAU
19
PER CAP SDA PAU
20
PER CAP SDA PAU
21
PER CAP SDA PAU
22
PER CAP SDA PAU
23
PER CAP SDA PAU
24
PER CAP SDA PAU
25
PER CAP SDA PAU
26
PER CAP SDA PAU
27
PER CAP SDA PAU
28
PER CAP SDA PAU
29
PER CAP SDA PAU
30
PER CAP SDA PAU
31
PER CAP SDA PAU

 August 2020
SUN MON TUE WED THU FRI SAT
1
PER CAP SDA PAU
2
PER CAP SDA PAU
3
KCG PER CAP SDA PAU
4
KCG PER CAP SDA PAU
5
KCG PER CAP SDA PAU
6
KCG PER CAP SDA PAU
7
KCG PER CAP SDA PAU
8
KCG PER CAP SDA PAU
9
KCG PER CAP SDA PAU
10
KCG PER CAP SDA PAU
11
KCG PER CAP SDA
12
KCG PER CAP SDA
13
KCG PER CAP SDA
14
KCG PER CAP SDA
15
KCG PER CAP SDA
16
KCG PER SDA
17
KCG PER SDA
18
KCG PER SDA
19
KCG PER SDA
20
KCG PER SDA
21
KCG PER SDA
22
KCG PER SDA
23
KCG PER SDA
24
KCG PER
25
KCG
26 27 28
AUR
29
AUR
30
AUR
31
AUR

 September 2020
SUN MON TUE WED THU FRI SAT
1
AUR
2
AUR
3
AUR
4
AUR
5
SPE AUR
6
SPE
7
SPE
8
SPE
9
DSX SPE
10
STA DSX SPE
11
STA DSX SPE
12
STA DSX SPE
13
STA DSX SPE
14
STA DSX SPE
15
STA DSX SPE
16
STA DSX SPE
17
STA DSX SPE
18
STA DSX SPE
19
STA DSX SPE
20
STA DSX SPE
21
STA DSX SPE
22
STA DSX
23
STA DSX
24
STA DSX
25
STA DSX
26
STA DSX
27
STA DSX
28
STA DSX
29
STA DSX
30
STA DSX

 October 2020
SUN MON TUE WED THU FRI SAT
1
STA DSX
2
ORI STA DSX
3
ORI STA DSX
4
ORI STA OCT DSX
5
ORI STA OCT DSX
6
ORI STA DRA OCT DSX
7
ORI STA DRA DSX
8
ORI STA DRA DSX
9
ORI STA DRA DSX
10
ORI DAU STA DRA
11
ORI DAU STA
12
ORI DAU STA
13
ORI DAU STA
14
ORI EGE DAU STA
15
ORI EGE DAU STA
16
ORI EGE DAU STA
17
ORI EGE DAU STA
18
ORI EGE DAU STA
19
LMI ORI EGE STA
20
NTA LMI ORI EGE STA
21
NTA LMI ORI EGE STA
22
NTA LMI ORI EGE STA
23
NTA LMI ORI EGE STA
24
NTA LMI ORI EGE STA
25
NTA LMI ORI EGE STA
26
NTA LMI ORI EGE STA
27
NTA LMI ORI EGE STA
28
NTA ORI STA
29
NTA ORI STA
30
NTA ORI STA
31
NTA ORI ST
 November 2020
SUN MON TUE WED THU FRI SAT
1
NTA ORI STA
2
NTA ORI STA
3
NTA ORI STA
4
NTA ORI STA
5
NTA ORI STA
6
LEO NTA ORI STA
7
LEO NTA ORI STA
8
LEO NTA STA
9
LEO NTA STA
10
LEO NTA STA
11
LEO NTA STA
12
LEO NTA STA
13
NOO LEO NTA STA
14
NOO LEO NTA STA
15
NOO AMO LEO NTA STA
16
NOO AMO LEO NTA STA
17
NOO AMO LEO NTA STA
18
NOO AMO LEO NTA STA
19
NOO AMO LEO NTA STA
20
NOO AMO LEO NTA STA
21
NOO AMO LEO NTA
22
NOO AMO LEO NTA
23
NOO AMO LEO NTA
24
NOO AMO LEO NTA
25
NOO AMO LEO NTA
26
NOO LEO NTA
27
NOO LEO NTA
28
PHO NOO LEO NTA
29
PHO NOO LEO NTA
30
PHO NOO LEO NTA

 December 2020
SUN MON TUE WED THU FRI SAT
1
PUP PHO NOO NTA
2
PUP PHO NOO NTA
3
HYD PUP PHO NOO NTA
4
GEM HYD PUP PHO NOO NTA
5
DLM GEM HYD MON PUP PHO NOO NTA
6
DLM GEM HYD MON PUP PHO NOO NTA
7
DLM GEM HYD MON PUP PHO NTA
8
DLM GEM HYD MON PUP PHO NTA
9
DLM GEM HYD MON PUP PHO NTA
10
DLM GEM HYD MON PUP NTA
11
DLM GEM HYD MON PUP
12
DLM COM GEM HYD MON PUP
13
DLM COM GEM HYD MON PUP
14
DLM COM GEM HYD MON PUP
15
DLM COM GEM HYD MON PUP
16
DLM COM GEM HYD MON
17
DLM URS COM GEM HYD MON
18
DLM URS COM GEM HYD MON
19
DLM URS COM GEM HYD MON
20
DLM URS COM GEM HYD MON
21
DLM URS COM
22
DLM URS COM
23
DLM URS COM
24
DLM URS
25
DLM URS
26
DLM URS
27
DLM
28
DLM QUA
29
DLM QUA
30
DLM QUA
31
DLM QUA

## Venus: Future Earth?

In terms of bulk properties, Venus is the most Earthlike planet in the solar system. The diameter of Venus is 95% of Earth’s diameter. The mass of Venus is 82% of Earth’s mass. It has a nearly identical composition.

But…the average surface temperature of Venus is 735 K (863˚ F) and the surface atmospheric pressure is 91 times greater than Earth’s—equivalent to the pressure 3,000 ft. below the ocean’s surface. The present atmosphere of Venus is composed of 96.5% carbon dioxide (CO2) and 3.5% nitrogen (N2), plus a number of trace elements and compounds.

Venus was not always so inhospitable. What happened?

The cratering record suggests that nearly all of Venus has been resurfaced within the last 300 – 800 Myr. Before that, Venus probably was much more hospitable, even habitable, perhaps. The Pioneer Venus large probe and infrared spectral observations from Earth of H2O and HDO (deuterated isotope of water) indicate that the deuterium-to-hydrogen ratio in the Venusian atmosphere is 120 – 157 times higher than in water on Earth, strongly suggesting that Venus was once much wetter than it is today and that it has lost much of the water it once had to space. (Hydrogen is lighter than deuterium and therefore more easily escapes to space.) In addition to deuterium abundance, measuring the isotopic abundance ratios of the noble gases krypton and xenon would help us better understand the water history of Venus. These cannot be measured remotely and requires at-Venus sampling.

Venus receives 1.92 times as much solar radiation as the Earth, and this was undoubtedly a catalyst for the runaway greenhouse effect that transformed the Venusian climate millions of years ago.

We know that CO2 is a potent greenhouse gas, but anything that increases the amount of water vapor (H2O) in the atmosphere leads to global warming as well. As do clouds.

Climate modeling shows us that that the hothouse on the surface of Venus today is due to CO2 (66.6%), the continual cloud cover (22.5%), and what little water vapor remains in the atmosphere (10.9%).

Interestingly, if all the CO2 and N2 in the Earth’s crust were somehow liberated into the atmosphere, our planet would have an atmosphere very similar to Venus.

Venus is the easiest planet to get to from Earth, requiring the least amount of rocket fuel. There is so much we still don’t understand about how Venus transformed into a hellish world, and we would be well-advised to learn more about Venus because it may inform us about Earth’s future as well.

Tessera terrain covers about 7% of the surface of Venus. These highly deformed landforms, perhaps unique in the solar system, may allow us to someday sample the only materials that existed prior to the great resurfacing event.

If living organisms ever developed on Venus, the only place they could still survive today is 30 miles or so above the surface where the atmospheric temperature and pressure are similar to the surface of the Earth.

Even four billion years ago, Venus may have been too close to the Sun for life to develop, but if it did, Venus probably remained habitable up to at least 715 Myr ago.

Now for the bad news. All main-sequence stars, including our Sun, slowly brighten as they age, and their habitable zones move outward from their original locations. Our brightening Sun will eventually render the Earth uninhabitable, certainly within the next two billion years, and our water could be lost to the atmosphere and then space within the next 13o million years, leading to a thermal runaway event and an environment similar to that of Venus. Human-induced climate change could make the Earth uninhabitable for humans and many other species long before that.

One indication that water is being lost to space and surface warming is occurring is water vapor in the stratosphere. The more water vapor that is in the stratosphere, the more water is being forever lost to space and the greater the surface warming. Careful and continuous monitoring of water vapor levels in the Earth’s stratosphere is important to our understanding of climate change on Earth.

To conclude, Arney and Kane write:

“Venus teaches us that habitability is not a static state that planets remain in throughout their entire lives. Habitability can be lost, and the runaway greenhouse is the final resting place of once watery worlds.”

References

Arney, G., & Kane, S. 2018, arXiv e-prints, arXiv: 1804.05889

Bézard, B., & de Bergh, C. 2007, J. Geophys. Res., 112, E04S07, doi: 10.1029/2006JE002794.

Ostberg, C., & Kane, S. R. 2019, arXiv e-prints,arXiv: 1909.07456

Sanjay S. Limaye, Rakesh Mogul, David J. Smith, Arif H. Ansari, Grzegorz P. Słowik, and Parag Vaishampayan. Astrobiology. Sep 2018.1181-1198. https://www.liebertpub.com/doi/10.1089/ast.2017.1783

Way, M.J. 2019, EPSC Abstracts, 13, EPSC-DPS2019-1846-1

Way, M. J., Del Genio, A. D., Kiang, N. Y., et al. 2016, Geophys. Res. Lett., 43, 8376

## Keith Bechtol at UW Space Place

We are so very fortunate here in southern Wisconsin to have evening public lectures the 2nd Tuesday every month of the year at the University of Wisconsin Space Place, expertly organized by Jim Lattis. On Tuesday, November 12th, Clif Cavanaugh (retired physics and astronomy professor at the UW in Richland Center) and I made the trek (as we often do) from Spring Green-Dodgeville to the Space Place in Madison. This month, we were treated to an excellent presentation by Keith Bechtol, an Observational Cosmologist in the Physics Department at UW-Madison. His topic was The Big Picture: Science with Astronomical Surveys. Keith is an early career scientist with a bright future. His presentation was outstanding.

I’d like to share with you some of the highlights.

Before the talk, which is mostly about the Large Synoptic Survey Telescope (LSST), currently under construction in Chile and expected to see first light in 2020, I asked Keith about whether LSST would be renamed the Vera Rubin Telescope as was announced at AAS 234 in St. Louis this past summer. As it turns out, Keith has been a vocal advocate for naming LSST after Vera Rubin, though no final decision has yet been made.

Before I get into notes from the talk, I wanted to share with you the definition of the word synoptic in case you are not familiar with that word. The Oxford English Dictionary defines the word synoptic as “furnishing a general view of some subject; spec. depicting or dealing with weather conditions over a large area at the same point in time.” But rather than the traditional meteorological definition, here we are referring to a wide-field survey of the entire night sky visible from Cerro Pachón in Chile, latitude 30˚ S.

Keith first talked about how astronomical imaging is currently advancing along two fronts. The first is high-resolution imaging, as recently illustrated with first image of the event horizon of a black hole from the Event Horizon Telescope, where an amazing resolution of around 25 microarcseconds was achieved.

In general, the larger the telescope aperture, the smaller the field of view.

A survey telescope, on the other hand, must be designed to cover a much larger area of the sky for each image.

Not only can a survey telescope detect “anything that changes” in the night sky, but it also allows us to probe the large-scale structure of our universe. Three still-mysterious entities that are known to affect this large-scale structure are dark energy, dark matter, and neutrinos. Keith indicates that “these names are placeholders for physics we don’t yet fully understand.”

Dark energy, which is responsible for driving galaxies apart at an accelerating rate, is unusual in that it maintains a constant density as the universe expands. And its density is very low.

Supernovae are a very useful tool to probe the dark-energy-induced accelerating expansion of the universe, but in any particular galaxy they are exceedingly rare, so by monitoring large areas of the sky (ideally, the entire sky), we can discover supernovae frequently.

The mass distribution of our universe subtly affects the alignment and shapes of distant galaxies through a phenomenon known as weak gravitational lensing. Understanding these distortions and correlations requires a statistical approach looking at many galaxies across large swaths of sky.

Closer to home, small galaxies that have come too close our Milky Way galaxy are pulled apart into stellar streams that require a “big picture” approach to discover and map. The dark matter distribution in our Milky Way galaxy plays an important role in shaping these stellar streams—our galaxy contains about ten times as much dark matter as normal matter.

With wide-field surveys, not only do we need to cover large areas of sky, but we also want to be able to see the faintest and most distant objects. That latter property is referred to as “going deeper”.

The LSST will provide a dramatic increase in light gathering power over previous survey instruments. The total number of photons collected by a survey instrument per unit time is known as the étendue, a French word, and it is the field of view (in square degrees) × the effective aperture (in m2) × the quantum efficiency (unitless fraction). The units of étendue are thus m2deg2. Note that the vertical axis in the graph below is logarithmic, so the LSST will have a significantly higher étendue than previous survey instruments.

The largest monolithic mirrors in the world are fabricated at the Steward Observatory Mirror Lab at the University of Arizona in Tucson. The largest mirrors that can be produced there are 8.4 meters, and LSST has one of them.

Remember the Yerkes Observatory 40-inch refractor, completed in 1897? It has held the record as the largest lens ever used in an astronomical telescope. Until now. A 61.8-inch lens (L-1) and a 47.2-inch (L-2) have been fabricated for use in the LSST camera.

LSST will utilize a camera that is about the size of a car. It is the largest camera ever built for astronomy.

The LSST camera will produce 3.2 gigapixel images. You would need to cover about half a basketball court with 4K TV screens to display the image at full resolution.

An image will be produced every 15 seconds throughout the night, every clear night, and each patch of sky will be reimaged every three nights. That is a HUGE amount of data! ~10 Tb of data each night. Fiber optical cable will transport the data from Cerro Pachón to the National Center for Supercomputing Applications in Urbana, Illinois, where it will be prepared for immediate use and made publicly available to any interested researcher. The amount of data is so large that no one will be downloading raw data to their local computer. They will instead be logging in to the supercomputer and all processing of the data will be done there, using open source software packages.

There are many data processing challenges with LSST data needing to be solved. Airplane, satellite, and meteor trails will need to be carefully removed. Many images will be so densely packed with overlapping objects that special care will be needed separating the various objects.

One LSST slide that Keith presented showed “Solar System Objects: ~ 6 million” and that piqued my interest, given my ongoing research program of observing stellar occultations by asteroids and trans-Neptunian objects for IOTA. Currently, if you endeavor to observe the highest probability occultation events from a fixed observatory location each night, you will be lucky to record one positive event for every ten negative events (no occultation). The reason for this is that our knowledge of the orbital elements of the small bodies of the solar system is not yet precise enough to accurately predict where stellar occultation events will occur. Gaia DR3, scheduled for the latter half of 2021, should significantly improve the precision of small body orbits, and even though LSST does not have nearly the astrometric precision of Gaia, it will provide many valuable astrometric data points over time that can be used to refine orbital elements. Moreover, it is expected that LSST will discover—with its much larger aperture than Gaia—at least 10 times the number of asteroids and trans-Neptunian objects that are currently known.

During the question and answer period after the lecture, I asked Keith what effect the gigantic increase in the number of satellites in Earth orbit will have on LSST operations (global broadband internet services provided by organizations like SpaceX with its Starlink constellation). He stated that this definitely presents a data processing challenge that they are still working on.

An earlier version of Keith’s presentation can be found here. All images in this article except the first (OED) come from Keith’s presentation and have not been altered in any way.

References

Bechtol, Keith, “The Big Picture: Science with Astronomical Surveys” (lecture, University of Wisconsin Space Place, Madison, November 12, 2019).

Bechtol, Ellen & Keith, “The Big Picture: Science and Public Outreach with Astronomical Surveys” (lecture, Wednesday Night at the Lab, University of Wisconsin, Madison, April 17, 2019; University Place, Corporation for Public Broadcasting, PBS Wisconsin).

Jones, R. L., Jurić, M., & Ivezić, Ž. 2016, in IAU Symposium, Vol. 318, Asteroids: New Observations, New Models, ed. S. R. Chesley, A. Morbidelli, R. Jedicke, & D. Farnocchia, 282–292. https://arxiv.org/abs/1511.03199 .

Oxford English Dictionary Online, accessed November 17, 2019, https://www.oed.com/ .