D e s e r t E x p o s u r e
April
2008
Constellation of the Month: Pyxis, the Compass
As the weather warms up on these spring nights, the smallish constellation Pyxis, the Compass, crosses our southern sky. This constellation was named by Nicolas Louis de Lacaille as Pyxis Nautica. Lacaille was a French astronomer who compiled a catalogue of 10,000 southern stars, called Coelum Astrale Stelliferum, that was published posthumously in 1763. This catalogue introduced 14 new constellations, including Pyxis. These were incorporated into the list of official constellations when they were defined by the International Astronomical Union (IAU) in 1930.
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Click the image for a larger map.
Low in the south in our April evening sky, Pyxis, the Compass, slides along the southern horizon. Even though it is on the edge of the Milky Way, there are few interesting objects in this smallish constellation. Mars is above, and Saturn above and to the left of Pyxis. |
The rationale behind this constellation is a little confusing. Since the Compass represents a nautical tool, some believe it is part of Argo Navis, one of Ptolemy's original 48 constellations. This huge constellation represented Jason and the Argonauts' ship, the Argo. Argo Navis was broken up into Carina (The Keel), Puppis (The Stern, or Poop Deck) and Vela (The Sails) when the constellations were codified.
Deriving from a single original constellation, Carina, Puppis and Vela share a common group of "Bayer designations" naming individual stars. There is an Alpha Carinae (Canopus), but no Alpha Puppis or Alpha Velorum. Similarly, there is a Gamma Velorum, but no Gamma Puppis or Gamma Carinae. But Pyxis has its own set of Bayer designations, so it is clearly not part of Argo Navis. Most likely, de Lacaille, being a technophile, simply wanted to represent a nautical compass in the sky.
The IAU, the same organization that ratified the boundaries of the constellations, also created a definition of a planet in 2006. The word "planet" comes from the Greek for "wanderer." The ancients could see the bright planets wandering through the night sky, and thought of them as gods traversing the universe to and from Mt. Olympus. There are only five classical planets: Mercury, Venus, Mars, Jupiter and Saturn. This was the roster until March 13, 1781, when Sir William Herschel announced the discovery with his telescope of Uranus out beyond Saturn.
There still seemed to be a hole between Mars and Jupiter, however. On Jan. 1, 1801, Giuseppe Piazzi, an Italian Monk, discovered Ceres to be orbiting in that gap. The next year, on March 28, 1802, German astronomer Heinrich Wilhelm Matthaus Olbers discovered Pallas, also between Mars and Jupiter. Juno was discovered in 1803 by German astronomer Karl L. Hardingand and Vesta, also by Oblers, in 1807. There would be no further discoveries of planets for 38 years. During this time, all four of these were considered planets.
Watch the Skies
(all times MDT)
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Dec. 8, 1845, marked the discovery of Astraea by German astronomer Karl Ludwig Hencke. The next year, Frenchman James Challis, using an orbit computed based on irregularities in Uranus' orbit, discovered Neptune on Sept. 23, 1846. This orbit was independently computed by John Couch Adams in England and Urbain Le Verrier in France.
A subsequent flood of asteroid discoveries between Mars and Jupiter caused astronomers to demote all of them to the status of minor planet. Clyde Tombaugh, who became a Las Cruces resident in his later years, discovered Pluto on Feb. 18, 1930. Since it was at the outer edge of the solar system, beyond the asteroid belt, Pluto was considered a full planet.
Fast-forward to 2001, when astronomers started to discover additional objects out beyond Pluto — some, like Eris, larger than Pluto. These trans-Neptunian objects (TNOs) caused astronomers to be once again faced with a proliferation of planets. Either all these TNOs, including Pluto, were planets, or none were. To resolve this problem, on August 24, 2006, the IAU voted a new definition that makes Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune full "planets." Ceres, Pluto and Eris were declared "dwarf planets" and all the rest "small solar system bodies." These include both asteroids (minor planets) and comets.
The Planets for April 2008
Mars will spend April drifting eastward among the stars of Gemini in our evening sky. Having already passed the meridian before sunset, Mars will sink down in the west each evening until it disappears over the west-northwestern horizon around 1:30 a.m. Mars' 6.3 second-of-arc disc is 90-percent illuminated, shining at magnitude -1.0 on April 15.
Saturn is moving westward (retrograde motion) among the stars of Leo, drifting slowly toward Regulus. Fading slightly from last month as the Earth pulls away from Saturn, it is magnitude 0.4 in the middle of the month. The ball of the planet is only 19.1 seconds-of-arc across, and the Rings are 43.3 seconds-of-arc across. They are tilted up 9.8 degrees to our line of sight with the southern face showing. Saturn is two-thirds of the way up in the southeastern sky at sunset, and sets around 4 a.m.
Still moving eastward among the stars of northeastern Sagittarius, Jupiter is in our morning sky rising around 2 a.m. By sunrise, Jupiter will be a third of the way up in the southern sky. Now at magnitude -2.3, Jupiter's disc is 39.1 seconds-of-arc across. While Jupiter's motion away from the Sun should make it better placed for telescopic observation, Jupiter is still very far south. This means we have to look through more distorting atmosphere to see the King of the Planets than we would if it were farther north.
Venus and Mercury are both too close to the Sun to be seen this month. They are on the opposite side of the Sun from us. Both are moving eastward, but Mercury is moving much more swiftly and will be in our evening skies in May. Venus will not return in the evening sky until late June. While we wait, take your telescope out and take a look at the other planets that are visible, and "keep watching the sky"!