D e s e r t E x p o s u r e
April
2009
Constellation of the Month: Puppis, the Stern
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Click the Image for a Larger Map
Puppis, the Poop Deck or Stern, was once part of the constellation Argo Navis, named for the ship that carried Jason and the Argonauts in their quest for the Golden Fleece. With the Milky Way passing through it, this constellation has many open clusters, including M46 and M47. This pair appears close together in the sky, but M47 is much closer to us than M46. |
In the first century AD, Greek astronomer Ptolemy listed the 48 classical constellations in our sky. Naturally, these constellations were only the ones visible from Greece. Ptolemy did not actually create them; he simply listed those commonly known to his fellow Greeks. Many were related to mythologies built around the Greek gods and goddesses. One well-known story that has been immortalized in books and movies is Jason and the Argonauts. Jason himself was never placed in the sky, but his ship, the Argo, was. Some of his crew — Hercules, Castor and Pollux — also made it into the sky, the latter two being the head stars in Gemini, the Twins.
Jason was sent on a quest for the Golden Fleece by Pelias, his half-uncle, who had killed most of the rest of his family in a bid to become king of Thessaly. Jason had been hidden by his mother, who said he'd been still-born. In reality, she sent Jason to the highly intelligent and educated centaur Chiron for safekeeping and education.
The adult Jason returned to Iolcus (the modern-day city of Volos), where he told Pelias that he was the rightful king. Pelias said Jason could have the throne if he brought back the Golden Fleece. Jason agreed and assembled an illustrious crew that included the Boreads (sons of Boreas, the North Wind), Hercules, Philoctetes, Peleus, Telamon, Orpheus, Castor and Pollux, Atalanta and Euphemus.
Jason and the Argonauts sailed off, making many landfalls and fighting many battles. In Colchis, Eros made Medea fall in love with Jason, and she used her magical powers to help him through the rest of the voyage. In the end, however, Jason broke his vow to love Medea forever and lost favor with Hera. He died a lonely and unhappy man when the rotting Argo fell on him as he was sleeping beneath it. The ship was placed into the sky as Argo Navis, the largest of Ptolemy's 48 classical constellations.
Argo Navis (or just Argo) is the only classical constellation not included in the 88 modern constellations. Our old friend, French astronomer Nicolas Louis de Lacaille, divided it into Carina (the keel of the ship), Puppis (the poop deck or stern) and Vela (the sails). When Argo Navis was split, its Bayer designations were also split: Carina got Alpha, Beta and Epsilon, while Vela has Gamma and Delta, and Puppis got Zeta, and so on.
Puppis is southeast of Canis Major (which is marked by the brightest star in the sky, Sirius). The southern Milky Way runs through it and there are a number of open star clusters in this constellation. Two of them, M46 and M47, are visible in the same binocular field. This pair is only 1.3 degrees apart and bright enough to be seen in even moderately dark skies. While this pair appears close together in our sky, they are not near each other in space.
M46 is almost the size of the Full Moon and is about 5,500 light years way. M47 appears just slightly larger, but is only 1,600 light years away — so even though they are almost along the same line of sight, they are 3,900 light years apart. M47 contains about 50 stars, with the brightest at magnitude 5.7. The whole cluster is magnitude 5.2. M46 has roughly 500 stars, and is magnitude 6.1. In front of it (though actually unrelated to M46) is a little planetary nebula, NGC 2438, which is only 1.1 minutes of arc across at magnitude 10.8.
The Planets for April 2009
With Venus out of our evening sky, we get a stand-in in the form of Mercury popping up into our evening sky for the last three weeks of the month. By mid-April, Mercury will be 6.1 seconds-of-arc across and will be a gibbous 72-percent illuminated at magnitude -0.9. The Messenger of the Gods has moved out from the far side of the Sun and is coming around toward us. By month's end, it will be a 24-percent illuminated crescent shining at magnitude 1.0. Since it will be closer to us, it will have grown in size to 8.9 seconds-of-arc across.
Saturn is also visible in the evening, about halfway up in the southeastern sky. With Saturn now past opposition, the Earth is pulling away from Saturn, making it grow smaller and fainter. At mid-month the Ringed Planet sets around 5 a.m.; through a telescope it is 19.2 seconds-of-arc across, shining at magnitude +0.6. The Rings are now tilted up 3.8 degrees and are 43.6 seconds-of-arc across, with the southern face showing.
The King of the Planets is moving slowly eastward in eastern Capricornus this month. Jupiter rises in the east-southeast around 3:45 a.m., making it still a poor target for telescopic observation. At midmonth Jupiter is magnitude -2.2 and is 36.2 seconds-of-arc across.
Mars rises around 5:15 a.m. It starts the month in Aquarius and then slides along the ecliptic into Pisces, where it meets up with Venus. In mid-April Mars is magnitude 1.2 and a paltry 4.2 seconds-of-arc across, making it still a poor telescopic target. The God of War will be only 10 degrees up in the east-southeast as it starts to get light.
Watch the Skies
(all times MDT)
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Venus is the last planet to rise before it gets light. Having graced our evening sky for many months, Venus is just starting its appearance in the morning. By mid-month, it will be magnitude -4.4 with a disc that is a thin 10-percent crescent, 50.3 seconds of arc across. Venus has just passed between the Earth and Sun and is now racing ahead of us. It will continue to shrink and become smaller as it pulls away. On April 18, it will be 6 degrees north of Mars, with Mars heading east and Venus racing west.
Venus will be covered up by the southwest edge of the Moon on April 22 around 6:13 a.m. The Sun will still be down, but the sky will be brightening. This disappearance is best observed with a telescope or binoculars; the exact time will vary depending on your location. By the time Venus reappears at 6:54 a.m., the Sun will be up, so it would be best to track the 8-percent crescent Moon until you see Venus again. While you are waiting for Venus to reappear, take a look at the stars and "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.