November 25, 2004

The Shortest Day, the Earliest Sunset

It may seem strange now, but in June, the sun didn't set until 8:45 p.m. It's six months later, and the sun sinks below the horizon shortly after 5 p.m.

Our number of daylight hours clearly has shrunk. On the first day of summer, it reached its annual maximum for Roanoke at 14 hours 44 minutes. In just three weeks from now on Dec. 21, it will have dropped to only 9 hours 35 minutes, our annual minimum.

The noonday sun is much lower in the sky than it was several months ago and casts long midday shadows. Blame these short days and low sun elevation on the 23-degree tilt of our planet's axis. On the first day of winter, the Earth's north pole slants its farthest away from the sun, giving us our shortest day.

You would expect that the year's earliest sunset occurs on that day. This is not so. The Earth's slightly squashed, noncircular orbit in combination with the polar tilt is the cause. Follow the sunset times listed in the paper, and you'll find that sunset in the Roanoke area Dec. 21 is about 5:06 p.m., while on Dec. 7 it is 4 minutes earlier.

Naturally, shorter days mean longer nights. Early evening in December is a time of transition for major stars in the sky.

In the east, there are the numerous bright stars of the cold winter months - Rigel, Betelgeuse, Aldebaran, Capella, Procyon, and the brightest star in the night sky, Sirius. Watch Rigel, Betelgeuse and Sirius as they rise. Observers often report views of twinkling starlight gone wild with displays of colorful flashes.

In the west-northwest are the Summer Triangle stars of Vega, Altair and Deneb, which were very apparent during late summer and early fall. Cygnus, the swan, of which Deneb is the brightest star, takes on one of its popular nicknames - the Northern Cross. This time of year it does resemble an upright cross hovering just above the mountain ridge tops.

After a four-month absence, there is a bright planet in our evening sky - Saturn. This ringed world is currently in the constellation Gemini just below its two bright stars Castor and Pollux. The near full moon will be an ideal locator Dec. 27 when it is situated between Saturn and Pollux.

A rare alignment of solar system bodies takes place on the morning of Dec. 7 for observers throughout Southwest Virginia. Look low in the southeast about 3:30 a.m., and you will see the crescent moon slowly inching toward the bright planet Jupiter. At 3:53 the moon will block, or occult, the planet. Finally, at 5 a.m., Jupiter will pop back into view on the moon's dark side. Amateur astronomers using small telescopes will find this event even more unusual because during the occultation, Jupiter itself will be occulting its moon Io.

The gulf between our moon and Jupiter in this line-of-sight arrangement is truly astounding. Even though massive Jupiter is 40 times the diameter of our moon, it is 1,800 times farther away, making it appear starlike next to the moon. Through binoculars you may be able to distinguish Jupiter's tiny disk.

Adding to this already striking scene, at 5:45 a.m., Venus shines as the dazzling "star" near the horizon with a much dimmer Mars nearby.

Throughout the month, Venus slowly pulls away from reddish Mars and, from our perspective, moves toward the sun as it approaches the far side of its orbit.

Dress warmly, go into the long and frosty night. Even from the light-polluted skies of Roanoke and Blacksburg, December's bright stars and planets put on a great show.