## The Lunar Equator

The equator of the Moon is defined by its rotational axis. It is the great circle contained by the plane that is perpendicular to the line connecting the north and south poles of the Moon.

Here is a list of named lunar features through which the Moon’s equator passes, from lunar east to lunar west on the part of the Moon visible from the Earth.

Remember, lunar longitude is opposite the direction in the sky. East longitude is the west/right side of the moon, and west longitude is the east/left side of the moon as viewed from the northern hemisphere of the Earth.

The diameter of each feature is included, followed by the depth of the feature, where available.

Wyld
Center: 98.101˚ E, 1.416˚ S
Range: 96.395˚ - 99.806˚ E, 3.121˚ S - 0.289˚N
Crater; libration zone
58 mi
James Hart Wyld (1913-1953), American rocket engineer
Dorsum Cloos
Center: 90.410˚ E, 1.149˚ N
Range: 90.387˚ - 91.144˚ E, 0.268˚ S - 2.576˚ N
Wrinkle ridge; libration zone
64 mi
Hans Cloos (1885-1951), German geologist
Mare Smythii
Center: 87.049˚ E, 1.709˚ S
Range: 80.941˚ - 92.719˚ E, 7.456˚ S - 4.496˚ N
Mare; libration zone; Smyth's Sea
232 mi, 3.1 mi
William Henry Smyth (1788-1865), English naval officer and astronomer
Schubert J
Center: 78.935˚ E, 0.012˚ S
Range: 78.579˚ - 79.292˚ E, 0.344˚ S - 0.320˚ N
Crater
12 mi
Friedrich Theodor von Schubert (1758-1825), German astronomer & geographer 
Jenkins
Center: 78.041˚ E, 0.372˚ N
Range: 77.418˚ - 78.663˚ E, 0.251˚ S - 0.994˚ N
Crater
24 mi, 1.9 mi
Louise Freeland Jenkins (1888-1970), American astronomer
Schubert X
Center: 76.750˚ E, 0.310˚ N
Range: 75.940˚ - 77.561˚ E, 0.501˚ S - 1.121˚ N
Crater
32 mi
Friedrich Theodor von Schubert (1758-1825), German astronomer & geographer
Nobili
Center: 75.949˚ E, 0.166˚ N
Range: 75.260˚ - 76.638˚ E, 0.523˚ S - 0.855˚ N
Crater
26 mi, 2.4 mi
Leopoldo Nobili (1784-1835), Italian physicist
Maclaurin X
Center: 68.708˚ E, 0.091˚ N
Range: 68.403˚ - 69.014˚ E, 0.214˚ S - 0.397˚ N
Crater
15 mi
Colin Maclaurin (1698-1746), Scottish mathematician
Maclaurin O
Center: 67.557˚ E, 0.135˚ S
Range: 66.873˚ - 68.241˚ E, 0.819˚ S - 0.548˚ N
Crater
23 mi
Colin Maclaurin (1698-1746), Scottish mathematician
Mare Spumans
Center: 65.303˚ E, 1.302˚ N
Range: 63.613˚ - 66.733˚ E, 1.062˚ S - 3.722˚ N
Mare
87 mi
The Foaming Sea
Webb C
Center: 63.833˚ E, 0.149˚ N
Range: 63.267˚ - 64.398˚ E, 0.247˚ S - 0.544˚ N
Crater
21 mi
Thomas William Webb (1807-1885), English astronomer
Sinus Successus
Center: 58.520˚ E, 1.124˚ N
Range: 56.519˚ - 60.188˚ E, 0.861˚ S - 2.872˚ N
Bay
82 mi
Bay of Success
Mare Fecunditatis
Center: 53.669˚ E, 7.835˚ S
Range: 40.771˚ - 63.340˚ E, 21.695˚ S - 6.112˚ N
Mare
429 mi, 1.1 mi
Sea of Fertility
Taruntius P
Center: 51.585˚ E, 0.060˚ N
Range: 51.473˚ - 51.696˚ E, 0.051˚ S - 0.172˚ N
Craterlet
5 mi, 0.9 mi
Lucius Tarutius Firmanus (fl. 86 B.C.), Roman philosopher, mathematician, and astrologer
Dorsum Cayeux
Center: 51.220˚ E, 0.763˚ N
Range: 50.922˚ - 52.000˚ E, 0.598˚ S - 2.113˚ N
Wrinkle ridge
59 mi
Lucien Cayeux (1864-1944), French sedimentary petrographer
Dorsa Cato
Center: 47.701˚ E, 0.213˚ N
Range: 46.605˚ - 49.599˚ E, 1.165˚ S - 2.233˚ N
Wrinkle ridges
87 mi
Marcus Porcius Cato (234-149 B.C.), Roman soldier, senator, and historian
Rima Messier
Center: 44.545˚ E, 0.756˚ S
Range: 43.357˚ - 45.581˚ E, 1.561˚ S - 0.015˚ N
Rille
62 mi
Charles Messier (1730-1817), French astronomer
Lubbock R
Center: 40.453˚ E, 0.167˚ S
Range: 40.060˚ - 40.845˚ E, 0.559˚ S - 0.225˚ N
Crater
15 mi
Sir John William Lubbock (1803-1865), English banker, barrister, mathematician, and astronomer
Maskelyne T
Center: 36.593˚ E, 0.040˚ S
Range: 36.507˚ - 36.678˚ E, 0.125˚ S - 0.046˚ N
Craterlet
3 mi
Nevil Maskelyne (1732-1811), English astronomer
Maskelyne A
Center: 34.089˚ E, 0.032˚ N
Range: 33.603˚ - 34.574˚ E, 0.453˚ S - 0.517˚ N
Crater
18 mi
Nevil Maskelyne (1732-1811), English astronomer
Mare Tranquillitatis
Center: 30.835˚ E, 8.349˚ N
Range: 16.924˚ - 45.490˚ E, 4.051˚ S - 19.375˚ N
Mare
544 mi
Sea of Tranquility
Rimae Hypatia
Center: 22.777˚ E, 0.340˚ S
Range: 19.690˚ - 25.975˚ E, 1.406˚ S - 0.672˚ N
Rilles
128 mi
Hypatia (c.370-415), Alexandrian philosopher, mathematician, and astronomer
Lade A
Center: 12.726˚ E, 0.161˚ S
Range: 11.773˚ - 13.680˚ E, 1.114˚ S - 0.793˚ N
Crater
35 mi
Heinrich Eduard von Lade (1817-1904), German banker and amateur astronomer
Lade B
Center: 9.796˚ E, 0.016˚ N
Range: 9.412˚ - 10.180˚ E, 0.368˚ S - 0.399˚ N
Crater
15 mi
Heinrich Eduard von Lade (1817-1904), German banker and amateur astronomer
Rhaeticus F
Center: 6.438˚ E, 0.060˚ S
Range: 6.134˚ - 6.742˚ E, 0.364˚ S - 0.244˚ N
Crater
11 mi
Georg Joachim Rheticus (1514-1574), Austria-born astronomer & mathematician
Rhaeticus
Center: 4.924˚ E, 0.032˚ N
Range: 4.192˚ - 5.657˚ E, 0.701˚ S - 0.764˚ N
Crater
30 x 27 mi, 1.0 mi
Georg Joachim Rheticus (1514-1574), Austria-born astronomer & mathematician
Rhaeticus L
Center: 3.484˚ E, 0.205˚ N
Range: 3.257˚ - 3.711˚ E, 0.022˚ S - 0.432˚ N
Crater
9 mi
Georg Joachim Rheticus (1514-1574), Austria-born astronomer & mathematician
Sinus Medii
Center: 1.027˚ E, 1.634˚ N
Range: 3.371˚ W - 5.551˚ E, 2.048˚ S - 4.641˚ N
Bay
178 mi
Bay of the Center
Mösting E
Center: 4.591˚ W, 0.178˚ N
Range: 5.189˚ - 3.992˚ W, 0.421˚ S - 0.777˚ N
Crater
27 mi
Johan Sigismund von Møsting (1759-1843), Danish banker, finance minister, and astronomy enthusiast
Sömmering
Center: 7.526˚ W, 0.193˚ N
Range: 7.987˚ - 7.065˚ W, 0.268˚ S - 0.654˚ N
Crater
17 mi, 0.8 mi
Samuel Thomas von Sömmering (1755–1830),German physician and anatomist
Lansberg
Center: 26.627˚ W, 0.312˚ S
Range: 27.266˚ - 25.988˚ W, 0.951˚ S - 0.327˚ N
Crater
24 mi, 1.9 mi
Philippe van Lansbergen (1561-1632), Dutch astronomer and mathematician
Mare Insularum
Center: 30.640˚ W, 7.792˚ N
Range: 39.195˚ - 22.153˚ W, 0.596˚ S - 16.345˚ N
Mare
318 mi
Sea of Islands
Oceanus Procellarum
Center: 56.677˚ W, 20.671˚ N
Range: 81.084˚ - 26.850˚ W, 16.266˚ S - 57.433˚ N
Mare
1611 x 353 mi
Ocean of Storms
Lohrmann D
Center: 65.273˚ W, 0.141˚ S
Range: 65.442˚ - 65.104˚ W, 0.310˚ S - 0.028˚ N
Crater
7 mi
Wilhelm Gotthelf Lohrmann (1796-1840), German selenographer
Rimae Hevelius
Center: 66.377˚ W, 0.809˚ N
Range: 67.849˚ - 63.582˚ W, 1.284˚ S - 2.956˚ N
Rilles
113 mi
Johannes Hevelius (1611-1687), Polish astronomer
Lohrmann
Center: 67.383˚ W, 0.440˚ S
Range: 67.898˚ - 66.867˚ W, 0.955˚ S - 0.075˚ N
Crater
19 mi, 1.0 mi
Wilhelm Gotthelf Lohrmann (1796-1840), German selenographer
Rimae Riccioli
Center: 73.071˚ W, 1.515˚ S
Range: 76.809˚ - 68.566˚ W, 4.754˚ S - 1.247˚ N
Rilles
249 mi
Giovanni Battista Riccioli (1598-1671), Italian astronomer
Schlüter P
Center: 85.208˚ W, 0.054˚ N
Range: 85.550˚ - 84.865˚ W, 0.289˚ S - 0.397˚ N
Crater; libration zone
12 mi
Heinrich Schlüter (1815-1844), German astronomer

Now that we’ve taken a tour of nearside features along the equator, let us turn to the lunar north and south celestial poles. As you know, the Earth’s north celestial pole (NCP) is currently located quite close to Polaris. However, on the Moon, the NCP is located in Draco near the Cat’s Eye Nebula (NGC 6543), about two-thirds of the way between Polaris and the center of the Head of Draco.

The Moon’s south celestial pole (SCP) is located in the constellation Dorado inside of the Large Magellanic Cloud (LMC). If you were stationed at the south pole of the Moon, you would see the Large Magellanic Cloud directly overhead at all times!

The Moon has many fascinating places, tempting us to explore. Some of them have quite interesting names. One of my favorites is Lacus Perseverantiae, Lake of Persistence. Its location is 62.0˚ E and 8.0˚ N. See if you can find it here. (Hint: under Layers : Overlays select Nomenclature, and under Settings select Show Graticule.) Have fun exploring!

References

Cocks, Elijah E.; Cocks, Josiah C. (1995). Who’s Who on the Moon: A Biographical Dictionary of Lunar Nomenclature. Tudor Publishers. ISBN978-0-936389-27-1.

1:1 Million-Scale Maps of the Moon, IAU/USGS/NASA. https://planetarynames.wr.usgs.gov/Page/Moon1to1MAtlas.

Virtual Moon Atlas 6.0 Pro. Computer software. https://ap-i.net/avl/en/start.

## Lunar Maria

António Cidadão, of Oeiras, Portugal, many years ago produced a wonderful set of images showing the location of each mare on the Moon. His website has not been updated since 1999 and the contact email address provided there is no longer valid, and even after a thorough Google search I can find no way to contact him to ask permission to link images here to his site. Even worse, because his hosting site is not secure (http: instead of https:), WordPress does not allow me to link directly to his images so I had to put copies into my media library. Please know that the images shown below are all copyrighted by António Cidadão.

Each image shows north is up and west is to the left. This is direction of increasing longitude and therefore west on the Moon, but in our sky, east is to the left. In other words, these annotated images of the Moon are correctly oriented as they would appear to the unaided eye in the sky in the northern hemisphere. In the rest of this article, we will use the moon-centric east-west convention that Cidadão indicates in his image diagrams.

Let’s take a look at each of the lunar maria from moon-west to moon-east. Their fanciful names were mostly given (and codified in 1651) by the Italian astronomer Giovanni Battista Riccioli (1598-1671). Riccioli chose names related to weather, as it was then believed that the Moon, the closest celestial body to the Earth, exerted an influence on the Earth’s weather. This is perhaps not at all surprising given that the phenomenon of tides had been known since antiquity.

Most of the nearside west portion of the Moon is covered by a mare that is so large that it is given a unique designation: Oceanus for “ocean”.

Oceanus Procellarum contains the famously bright crater Aristarchus and the associated Aristarchus Plateau. In the image above you will notice what appears to be a tiny mare close to the limb of the Moon west of the southern part of Oceanus Procellarum. This is the lava-flooded crater Grimaldi.

South of Grimaldi and straddling the lunar limb is Mare Orientale. It is difficult to see because most of it is on the lunar farside, though libration can sometimes bring its oblique visage into view. The name Orientale, meaning “eastern”, describes its location on the eastward-facing limb of the Moon as seen from Earth, rather than its westward direction as seen from the surface of the Moon.

Mare Humorum is located just south of Oceanus Procellarum. It is round and inviting, though no spacecraft has ever landed there.

Mare Nubium is east of Mare Humorum. The large crater Bullialdus flanks the western edge of Mare Nubium, and Rupes Recta (the “Straight Wall”) flanks its eastern edge.

Mare Cognitum lies between Mare Nubium and Oceanus Procellarum. It was named in 1964 after the Ranger 7 probe took the first U.S. close-up pictures of the Moon’s surface prior to crashing there.

Mare Insularum is north of Mare Cognitum. Its current name was bestowed upon it in 1976 by lunar geologist Don Wilhelms (1930-). The crater Kepler on its western edge separates Mare Insularum from Oceanus Procellarum. The crater Copernicus is on the northeast side of its western lobe.

Mare Vaporum is the mare closest to the center of the Moon’s nearside. The bright crater Manilius lies towards its northeastern edge and the volcanic crater Hyginus and its associated rille (Rima Hyginus) are immediately to its south.

Mare Imbrium was created 3.9 billion years ago when an asteroid some 150 miles across crashed into the Moon. This ancient feature is so large that it forms the right eye of the “Man in the Moon” we see when looking at a full or nearly full moon with our unaided eyes.

Mare Frigoris lies north and northeast of Mare Imbrium. The dark crater between them is Plato. It is the mare closest to the north pole of the Moon.

Now we begin our tour of the eastern hemisphere of the Moon’s nearside. Mare Serenitatis has the distinction of being the landing site of the last human mission to the Moon, Apollo 17, in 1972. It was also the landing site of the Soviet unmanned spacecraft Luna 21 just one month later.

Mare Tranquillitatis is perhaps the most famous of the lunar maria, as it was there that humans first set foot on the surface of the Moon in 1969. The Apollo 11 landing site is located near its southwest corner.

Mare Nectaris lies south of Mare Tranquillitatis. This small, isolated, and nearly circular mare sports a prominent crater, Theophilus, at its northwest corner.

East of Mare Nectaris lies Mare Fecunditatis. Superposed upon Mare Fecunditatis is the striking crater pair Messier and Messier A with two prominent rays evocative of a comet’s tail. Named after the famous French comet hunter Charles Messier (1730-1817), these craters and their associated rays were formed from a grazing impact from the east.

Mare Crisium is a round and isolated mare that makes it easy to remember why it is called the “Sea of Crises”. The Soviet Luna 24 unmanned sample return mission landed there in 1976. The six ounces of lunar materials it brought back to Earth are the last lunar samples scientists have received.

Mare Anguis lies just northeast of Mare Crisium and is called the “Serpent Sea” for its serpentine shape rather than the more fanciful name “Sea of Serpents” referred to by some science fiction authors.

Mare Undarum lies southeast of Mare Crisium. Its uneven texture and lack of uniform smoothness appears to justify its name as “the sea of waves”.

Mare Spumans lies south of Mare Undarum and east of Mare Fecunditatis. The bright crater Petit on the western side of this tiny mare evinces a bit of foam on “the foaming sea”.

Mare Australe hugs the southeastern limb of the lunar nearside. Though obliquely viewed from Earth and wrapping around to the lunar farside, favorable libration makes it visible in its entirety on occasion.

Mare Smythii on the eastern limb of the Moon is one of two lunar maria named after people. The lucky honoree is English hydrographer and astronomer William Henry Smyth (1788-1865). The lunar equator passes through Mare Smythii.

Mare Marginis lies east of Mare Crisium, right along the lunar limb. The crater Goddard on the northeast side of Mare Marginis exhibits bright deposits on its northeastern side. This crater and its associated deposits can only be seen from Earth during favorable librations.

Mare Humboldtianum lies along the northeastern limb of the Moon and is the other lunar mare named after a person. The German astronomer Johann Heinrich von Mädler (1794-1874) named this feature after German geographer and explorer Alexander von Humboldt (1769-1859).

This completes our tour of the 21 maria on the nearside of the Moon.

References

António Cidadão’s Home-Page of Lunar and Planetary Observation and CCD Imaging, Moon-“Light” Atlas.  Retrieved 22 April 2020.

Ewen A. Whitaker, Mapping and Naming the Moon: A History of Lunar Cartography and Nomenclature (Cambridge University Press, 2003).

## BepiColombo Passes Earth

The BepiColombo spacecraft flew by the Earth last night, the first of nine gravity-assist maneuvers it will make to slow it down so that it can go into orbit around the planet Mercury on 5 December 2025. This was the only Earth gravity assist. There will be a Venus flyby later this year and next year, and six Mercury flybys from 2021-2025.

BepiColombo passed 7,877 miles over the South Atlantic Ocean at 0425 UT on 10 April 2020 at its closest approach to Earth, and I was able to image it from my backyard observatory in Dodgeville, Wisconsin at 0600 UT at a distance (range) of 21,760 miles.

North is up and East to the left in the video frame, so BepiColombo is moving in a northwesterly direction. The two stars in the field are 3UC 145-134561 (12.2m, north) and 3UC 144-138354 (12.7m, south). The predicted equatorial coordinates (epoch of date) at 0600 UT from JPL Horizons were α = 11h 38m 03.90s, δ = -18° 08′ 25.4″. Please note when using JPL Horizons to generate ephemerides for spacecraft and minor planets passing close to the Earth that you should use the ICRF coordinates (astrometric) and not the apparent coordinates. They can be significantly different!

The integration time in the video above is 7.5 frames per second, or 0.13 second per frame. The field size is 17 x 11 arcminutes.

Here’s the video light curve of BepiColombo as it passed through the field. It was fairly constant in brightness with no obvious variability amidst the noisy measurements.

## 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.

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.

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.