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Constellation of the Month: Equuleus, the Little Horse

Our constellation of the month is Equuleus, the Little Horse. At a meager 72 square degrees, only the constellation of Crux is smaller. The brightest star in this constellation is only magnitude 3.9. You might think that this is another of Johannes Hevelius' inventions, but actually this is a classical constellation, listed by Ptolemy in the Almagest, dating to the second century AD. The stars in Equuleus were first mapped by the Greek astronomer Hipparchus (146-127 BC) in the first-ever star catalog, containing about 850 stars. He may also have been the actual creator of Equuleus. The truth is lost in history, along with the exact mythology of the Little Horse.

Click image to enlarge

Looking about halfway up in the southeastern sky, the small constellation of Equuleus is visible between Delphinus and Pegasus. The brightest star, Kitalpha, is only magnitude 3.9, so this will be a hard constellation to pick out.

One story is that Equuleus represents the foal Celeris (meaning "swiftness" or "speed"). Equuleus was either the half-brother or child of Pegasus, the Winged Horse, and was given to Mercury by Castor. A different story has Neptune striking his trident, creating Equuleus during a contest between the sea god and Athena to see who would be dominant. The original constellation name in Arabic was Al Faras al Awwal, or in Latin, Equus Prime, First Horse.

Since this constellation covers such a small area of the sky, there are few objects to talk about — a scattering of galaxies and a few double stars and faint variable stars.


Our Neighborhood

Since our constellation of the month merits such brief treatment, let's look at the near sky, and the size of our solar system. While there are many tables showing the distances of the planets from the Sun, the best way to understand our solar system is to build a small model of it.

Start out with a piece of calculator tape, about two arm lengths' worth. Mark one end "Sun" (do not write it too big — you will find out why later). The other end gets marked "Neptune/Pluto." Remember that Pluto is probably a former moon of Neptune that was knocked out of its orbit millions of years ago. The orbit of Pluto crosses that of Neptune and sometimes Neptune is the farthest out and sometimes Pluto is. Right now, it is Pluto, but the International Astronomical Union declared Pluto a dwarf planet this year so it really does not count, anyway.

Watch the Sky
(all times MDT)

Sept. 3, 8:32 p.m. — Last Quarter Moon

Sept. 11, 6:44 a.m. — New Moon (partial solar eclipse — not visible here)

Sept. 19, 10:48 a.m. — First Quarter Moon

Sept. 22, 3 a.m. — Mercury 0.08 degrees north of Spica

Sept. 23, 3:51 a.m. — September Equinox

Sept. 23, 5 p.m. — Venus greatest brilliancy

Sept. 26, 1:45 p.m. — Full Moon

If you fold the tape in half the long way, you can mark the crease, the middle of the distance scale of the solar system, as "Uranus," showing that the outer half of the solar system is empty space. Now fold the "Sun" up to "Uranus." This crease represents the orbit of Saturn. Fold Saturn up to the Sun, and the newly created crease is the orbit of Jupiter. From here out to Neptune is the realm of the gas-giant planets. Making another fold, we have the home of the asteroids, between Jupiter and Mars. From here to the Sun we have the realm of the rocky planets.

Folding the asteroid belt up to the Sun gives you the location of Mars. You will notice that the distance between planets is getting closer. Fold "Mars" to the Sun and the latest crease is our home, Earth, a bare 93 million miles from the Sun. Folding the "Earth" to the Sun, you can see the planets really bunching up as we get toward the Sun, because this crease is where Venus resides. It is going to be hard to make the final fold, but when "Venus" is folded over to the Sun, the final crease is where Mercury runs its 88-day race around the Sun. Did you leave enough room to write in poor Mercury's name?

Out beyond Pluto is the Oort Cloud, a home of all the comets that might one day get knocked into the inner solar system and grace our skies with a long, arcing tail in our morning or evening sky. On the scale of our model solar system, this cloud starts 10 feet from the Sun and extends out about two miles! Now you have a good picture of our solar system's scale.


The Planets for September 2007

Jupiter has replaced Venus as the bright planet in the western sky as it gets dark. Drifting eastward among the stars of Ophiuchus, the King of the Planets is magnitude -2.1 as it shrinks to 36.6 seconds-of-arc across at mid-month. Earth is pulling away from Jupiter, making it appear smaller and fainter. This will be the last month this year that you can get a good view of Jupiter, which is a third of the way up in the southwestern sky as it gets dark and sets by 11 p.m.

As midnight rolls around, Mars pops up on the east-northeastern horizon. Shining at magnitude 0.1, Mars is 8.8 seconds-of-arc across. It is now entering part of this apparition when it is big enough to view with a telescope and see some of the features. Some may not be visible yet, as a dust storm is blotting out large parts of the Martian disc. Mars starts the month in Taurus, but slips into Gemini just before the month ends.

Venus comes up about two hours before the Sun. Starting the month in Cancer, Venus slides into Leo on Sept. 20. Having come out from between the Earth and Sun, Venus is a slowly fattening crescent, 20-percent illuminated at midmonth, when it will be magnitude -4.5 and 43.2 seconds-of-arc across.

Saturn is in Leo, below Venus but very close to the bright star Regulus. Rising in the east a little after 5 a.m., Saturn is magnitude 0.60, and the ball of the planet is 16.3 seconds-of-arc across. The rings have been closing, and are now tipped only 9.5 degrees to us with the southern face showing. With less of its rings visible, Saturn will be a little fainter than it has been for the last few years. The rings are 36.9 seconds-of-arc across.

The autumnal equinox (Northern Hemisphere) will occur Sunday, Sept. 23, at 3:51 a.m. MDT. At this instant, the center of the Sun passes through the celestial equator. On that day, the time of sunrise and sunset will be the same no matter what latitude you are at. It is also when day and night have equal lengths, 12 hours apiece. This day marks the beginning of the season of autumn. The nights will be getting longer from now until December, giving you more time to "keep watching the sky"!


An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.


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