D e s e r t E x p o s u r e March 2010

Constellation of the Month: Carina, the Keel

If you look just above the southern horizon around 7 p.m., you will see a bright star twinkling just five degrees above the horizon. This is Canopus, the brightest star of Carina, our constellation of the month. Carina, the Keel, is the bottom part of what was originally Argo Navis, the ship Argo, sailed by Jason and the Argonauts in their epic mythological adventure. Argo Navis is the only one of the 48 constellations listed by the Greek astronomer Ptolemy during the second century AD in his "Great Treatise," the Almagest. This constellation stood intact until Nicholas Louis de Lacaille decomposed it in 1752.

Click the image for a larger map

The two brightest stars in the sky appear in the March sky: Sirius, in Canis Major, and Canopus, in Carina. The section of Carina we can see from the desert southwest is just a small portion of the total constellation. While we do get to see Canopus, one of the most interesting stars along with its attendant nebula, Eta Carinae, never comes over our horizon.

The original Argo Navis was a huge constellation, and de Lacaille split it into three constellations, one on top of the other. The topmost is Vela, the Sails; below it is Puppis, the Poop Deck; and the bottom is Carina, the Keel. The Bayer designations assigned to Argo Navis were not reassigned when it was split up, so Carina got stars Alpha, Beta and Epsilon. Alpha Carinae is the previously mentioned Canopus, the second-brightest star in the sky after Sirius.

Canopus' brightness is remarkable since it is 310 light-years away. The brightest star in our sky, Sirius, is only 8.6 light-years away, so Canopus must be intrinsically much brighter than Sirius. In fact, Canopus is 600 times brighter than Sirius, or 15,000 times brighter than our Sun. If Canopus were in our Solar System, it would extend out almost to the orbit of Mercury and would stretch 33 degrees across in our sky. Of course the Earth would be fried, so it is just as well we have our own Sun!

The name "Canopus" has an uncertain derivation. One supposed origin is from the name of the pilot of Menelaus' ship on its mission to retrieve Helen of Troy after she was taken by Paris. A different origin is from the Egyptian for "Golden Earth," as Canopus would appear somewhat reddish through the atmosphere just as the Sun does at sunrise or sunset. An ancient Egyptian port, named Canopus, existed near the mouth of the Nile River.

Many early spacecraft had special sensors to determine the orientation of the spacecraft. One sensor pointed at the Sun. The other, aimed 90 degrees away from the ecliptic, pointed at Canopus when the spacecraft was properly oriented. Canopus was an easy target because of its brightness.

Not only does Carina have the second-brightest star in the sky, but there is also a fourth-magnitude "star" catalogued by Edmond Halley in 1677, Eta Carinae. In 1730, this star brightened considerably, to second magnitude. By 1782, Eta had faded back to fourth magnitude. It flared again from 1835 to 1855, reaching its brightest in 1843 at magnitude -1, brighter than Canopus. It then faded from visibility, only to reappear in the 1990s, ending that decade with a sudden doubling of its brightness.

Eta Carinae is one of the most massive stars in the sky, containing some 120 solar masses. With such a high mass comes a great brightness, some 4 million times that of the Sun. It is between 7,500 and 10,000 light-years away, which is a good thing since astronomers expect that it will go supernova sometime in the next 300,000 years.

This star is accompanied by the Homunculus (from the Latin meaning "Little Man") Nebula, which surrounds it. This nebula is made up of gas and dust blown out from Eta Carinae during the 1841 outburst. The Homunculus Nebula looks like two balloons touching each other, with Eta Carina sitting at the point of contact.

The Homunculus itself sits in a much larger bright nebula called the Carina Nebula (NGC 3372), a huge cloud of dust and gas that contains several open star clusters. It is four times larger than the Orion Nebula, and even brighter. It is not as well known because it is in the far southern sky, and never peeks over our southern horizon.

The Planets for March 2010

Venus is now firmly entrenched in our evening sky, but Mercury will also make an appearance at the end of the month. Starting around March 22, Mercury will become visible on our western horizon. Each succeeding night will see it appear higher in the sky, moving upward toward the much brighter Venus. Mercury will never catch the Goddess of Love, but will come close at the end of the month and into early April. On March 31, Mercury is magnitude -0.9 and its disc is 6.2 seconds-of-arc across. Mercury is 70% illuminated at the end of the month and will become a crescent as we move into April.

Venus is still fairly low on our western horizon as it gets dark. At the end of the month its disc will be 95% illuminated and 10.5 seconds-of-arc across. It will shine at magnitude -3.9 and set just before 8 p.m. Venus starts the month in Aquarius, slides into Pisces and nicks the corner of Cetus, before it returns to Pisces and finally slips into Aries by month's end.

Also in our evening sky is Mars, spending the whole month in Cancer. Now well past opposition, the God of War's disc is shrinking, at 9.3 seconds-of-arc across on March 15. Mars sets around 3 a.m. and shines at magnitude +0.1.

Watch the Skies

(all times MST / MDT)

March 7, 8:42 a.m. — Last Quarter Moon

March 14, 2 a.m. — Daylight Savings Time begins

March 15, 3:01 p.m. — New Moon

March 20, 11:32 a.m. — March Equinox

March 21, 7 p.m. — Saturn at opposition

March 23, 5 a.m. — First Quarter Moon

March 29, 8:25 p.m. — Full Moon

Saturn is in opposition on March 22, so it is visible all night. Shining at magnitude +0.5, the Ringed Planet spends the month in Virgo, drifting slowly westward. Saturn's disc is 19.4 seconds-of-arc across at midmonth, but the Rings are 44.1 seconds-of-arc across and tilted down 3.4 degrees with the northern face showing. The planet appears at its maximum size for this year.

Jupiter will be back next month in the morning sky.

The March equinox will be March 20 at 11:32 a.m., when the Sun crosses the celestial equator going north. It marks the beginning of astronomical spring in the northern hemisphere and autumn in the southern hemisphere. So get ready for warmer weather 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.

Return to Top of Page