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The bumpy road of driving a small-town cab

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About the cover


D e s e r t   E x p o s u r e    March 2008

Constellation of the Month: Canis Minor, the Smaller Dog


Orion, the Hunter, has been gracing our winter skies with his brilliant stars, like Betelguese, Bellatrix and Rigel, his striking belt and the gorgeous Orion Nebula. But winter is wearing thin and Orion is heading west, to get lost in the evening twilight in another month or so. Following behind him are his two hunting dogs, Canis Major, with the brightest star in the sky, Sirius, and Canis Minor.

Canis Minor, the Smaller Dog, is a small constellation created in modern times around the seventh-brightest star in the sky, Procyon, and the third-magnitude star Gomeisa. The Greeks considered Orion to have only one dog, what we now call Canis Major. Procyon, which in Greek means "before the dog," was so named because it rose about an hour before Sirius. Procyon is magnitude 0.40, a main-sequence star only twice the size of our Sun. Procyon's brightness in our sky comes not from being intrinsically bright, but from the fact that it is only 11.4 light-years away.

Click the image for a larger map


Orion and his two hunting dogs dominate the winter sky. We are facing south and looking up about two-thirds of the way to the zenith. Canis Major is marked by the bright star Procyon, which appears as a solitary bright star in an empty area east of Milky Way-enshrouded Orion. Procyon has a companion, but it is not visible in small telescopes.

While Procyon appears to be a single star, it actually has a white dwarf companion star that orbits it every 40 years. Procyon B, as the companion is called, orbits at an average distance about the same as Uranus is from our Sun. But its orbit is quite eccentric, going as close as a Saturnesque distance and out well beyond the orbit of Uranus.

Procyon B is the core of a dead star, only 5,300 miles in diameter. It is composed of degenerate matter — atomic nuclei swimming in a sea of electrons stripped from those atoms. This allows the matter to pack tightly together, giving it a very high density. Procyon B is only twice the size of our Moon, but 16 million times more massive, a total of 0.6 solar masses. It is still glowing from its own internal heat, left over from when it was a star.

Procyon B is another example of an object that was predicted to exist before it was actually discovered. "Proper motion" — how we see a star move across our sky — differs from the star's actual motion in space because of perspective: A star moving at right angles to our line of sight will have a much larger proper motion than the same star moving at the same speed but directly along our line of sight. In 1840, Procyon's proper motion was determined to be not a straight line as with most stars, but to have a wiggle with a period of about 40 years. In 1861, Arthur Julius Georg Friedrich von Auswers (1838-1915) used this wiggle to deduce that there was a massive, almost invisible, companion orbiting Procyon. John M. Schaeberle (1853-1924) finally observed Procyon B in 1896 with the 36-inch refractor at Lick Observatory. Procyon B is very close to the brilliant Procyon A, but at least 15,000 times fainter; it was lost in the glare of Procyon A until a large enough telescope was built that could separate it out.

The Planets for March 2008


Mars and Saturn both grace our evening skies this month. Mars starts in eastern Taurus, but quickly slips into Gemini for the rest of the month. Mars continues to shrink, only 7.9 seconds-of-arc across at mid-month. The Earth keeps pulling away from the slower-moving Red Planet. Because of the angle at which we view Mars, its disc is 90-percent illuminated while it shines at magnitude -0.4. On March 29, Mars will be a third of a degree from the third-magnitude star Mebusta, at the waist of Castor in Gemini. Mars sets around 3 a.m.

Saturn remains in Leo, moving slowly toward Regulus. Saturn is just past opposition, so it is already up in the eastern sky at sunset and sets just before sunrise. Sustaining its brightness at magnitude 0.2 in the middle of the month, the Ringed Planet will now start getting fainter as we pull away from it, just as we are doing with Mars. Saturn's ball is only 19.8 seconds-of-arc across with the Rings tilted up 9.1 degrees to our line of sight, with the southern face showing, and 44.9 seconds-of-arc across.

Jupiter pops up over the horizon around 4 a.m. Now past the Teapot asterism of Sagittarius, the King of Planets is moving eastward in that constellation. Jupiter is slowly brightening, at magnitude -2.1, and is 35.7 seconds-of-arc across. Jupiter is still in the southernmost part of the ecliptic, which keeps it from moving upward more quickly in our sky for better viewing.

Venus and Mercury are the final two planets to come up in our morning sky. Mercury was heading westward on the morning of Feb. 26, as it passed the eastward-moving Venus. Mercury reaches its farthest point from the Sun on March 3, then turns around and starts chasing down Venus. By March 23, Mercury overtakes Venus, passing 1.0 degree south of it. Both will be low in the east-southeast just before sunrise. Venus and Mercury start the month in Capricornus and slide into Aquarius 10 days later. By month's end, Mercury makes it into Pisces with Venus close on its tail. At mid-month Venus is magnitude -3.9 and 10.8 seconds-of-arc across; its disc is 93 percent sunlit. Mercury is magnitude -0.1, only 6.0 seconds-of-arc across, but its disc is 73 percent sunlit. Both rise around 5:30 a.m. MST at the beginning of the month. By the end of the month, they will rise around 6:20 a.m. on daylight-savings time, which begins March 9.

The season of spring begins in the Northern Hemisphere on March 19 at 11:49 p.m. This is the Vernal Equinox, when the Sun crosses the Equator, heading north for what will be our summer. The Full Moon occurs just 13 hours later. This sets the stage for an early Easter this year, though the Catholic Church uses a fixed date for the Equinox (March 21) and the "ecclesiastical moon," which is based on tables created by the church. Easter is the next Sunday after the first Full Moon after the Vernal Equinox. The result is that Easter this year is on March 23 for most Christians, but Eastern Orthodox Christians use a different rule, making their Easter April 27. So enjoy Mercury and Venus in the morning sky before you start your Easter-egg hunt, 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.


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