SJAA Ephemeris January 2006 | SJAA Home | Contents | Previous | Next

The Shallow Sky

Rays of Light in the Dead of Winter

Akkana Peck


The Stardust spacecraft returns on January 15. “Cross your fingers for a soft landing!”



Mars has moved into Aries and is high in the sky at sunset and visible through most of the prime observing hours. It's quite a bit smaller now -- 12" as the year opens, shrinking to under 10" by month's end, about half the size it showed a few months ago at opposition. It will also be noticeably gibbous: it'll be easy to see the shadow along one edge instead of the full disk of the planet. But there's still plenty of detail to be seen, so don a warm jacket and don't give up on Mars quite yet.

Saturn reaches opposition on January 27th. It's observable all evening and shows a ring tilt of about 18 degrees. That's considerably less than last year: Saturn's rings are gradually closing, year by year as viewed from our vantage point, and by 2009 we will see them edge on.

But that's not for years. This year, for the next few months you can enjoy all the usual features of Saturn's ring system: the two bright rings, designated as A and B (the A ring is the outer one), with the dark Cassini division separating them; the dim grey C ring (also called the "Crepe ring") inside the A ring and closer to the planet; and, for sharp eyed observers on very steady nights, the slim dark gap near the outer edge of the A ring, sometimes called the Encke gap (though it was observed by several people prior to Encke).

And don't forget about its moons: most telescopes will show anywhere from four to six of them, and larger telescopes might show seven. We've been learning a great deal about Saturn's moons over the past year, thanks to the wonderful images sent back from the Cassini spacecraft; don't forget to take a look at them yourself the next time you're observing the ringed planet.

Venus and Mercury are both too close to the sun to observe most of January, though sharp-eyed observers might catch the pair just after sunset at the very beginning of the month. Venus reaches inferior conjunction (it's between us and the sun) on the 13th, while Mercury is at superior conjunction (the other side of the sun) on the 26th.

Jupiter is mostly a morning object, rising at a few hours past midnight. Of course, its bands and moons are wonderful at any hour.

Uranus, Neptune, and Pluto are all too close to the sun for good observing this month.

Starting at about 6pm on the 9th, the gibbous moon passes through the Pleiades. Alas, it won't occult the brightest stars of the cluster, but it will pass over plenty of faint stars, and should be a sight well worth catching.

On January 15, the Stardust probe comes home to earth. Dr. Scott Sandford of NASA Ames has spoken at a couple of past SJAA meetings to tell us about this very interesting project, which a year ago this month successfully collected dust from the tail of comet Wild 2 (pronounced "Vilt 2"). Now Stardust is bringing those bits of comet tail back down to earth so they can be analyzed to see what they tell us about the origin of our solar system. Stardust's sample return capsule will parachute down to a landing in Utah's Great Salt Lake desert at about 3:15 am. Cross your fingers for a soft landing!

Finally, the Earth reaches perihelion on January 4th: in other words, that's when our almost, but not quite, circular orbit brings us closest to the sun.

Closest? In the dead of winter?

Indeed. You probably already know that our seasons aren't caused by closeness to the sun: they're caused by the 23-degree tilt of the Earth's axis. (You'll see that 23-degree angle on star maps as the angle between the equator and the ecliptic.) Right now, during our Earth's northern hemisphere is tilted away from the sun, so the sun's rays strike us at an angle, instead of from nearly overhead as they do in summer. This means we receive much less light and heat in winter than in summer, even though we're about three million miles closer to the sun.

Does this mean that southern hemisphere seasons are more extreme than ours, since in the southern hemisphere, it's midsummer when we reach perihelion?

Well, sort of. But the southern hemisphere has a lot more ocean than then northern hemisphere. All that water is more resistant than land to temperature changes (that's the reason that coastal areas like our own tend to be milder in climate than inland areas). So the percentage of ocean tends to offset the effect of perihelion and aphelion (the time when we're farthest from the sun), and the result is that there isn't very much difference between the severity of the seasons in Earth's northern and southern hemispheres.


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