January 2006 Sky from the Keeble Observatory
It’s a new year – the 2006th since the (arbitrary) beginning of our current Gregorian
calendar cycle, or approximately the 4.5 billionth since the planets formed. Let’s
look more closely at what those two statements mean.
Earth orbits a middle class, middle aged star called the Sun. From Newton’s “Law”
of Universal Gravitation, we can show that the period of the orbit, i.e. the time
to go completely around the Sun with respect to the distant stars, depends only
on the mass of the Sun and the size of the orbit. Our orbit is almost circular,
actually an ellipse that has us about 1% closer to the Sun in January than in June.
A simple back of the envelope calculation shows that this orbit takes about 31.6
million seconds – OK, that’s just one year – but what’s a second?
The second used to be defined as 1/86400 of a “mean solar day” – but that’s just
a circular definition for 24 hours (each of which has 3600 seconds). A second is
now defined in terms of the frequency of a particular color of light from an atom
of the element cesium. That frequency has not changed since there were cesium atoms
to emit that light, but the length of a day has changed since the Earth first formed.
The biggest change comes about because the Moon and Sun raise tides, and in turn
pull on the rotating Earth in such a way as to slow it down. 4 billion years ago
a day was about 10 hours, now it’s about 24 hours. But, beyond that effect, Earth
is just not a stable rotating top. You may have heard or read about the introduction
of a “leap second” in December to bring the atomic clocks back in sync with the
solar day. That happens every few years, though some “leap seconds” are subtracted
rather than added.
Additionally, we have to be precise in defining a day! The 24 hour day is defined
in terms of the apparent position of the Sun. A “solar day” is the time for the
Sun to return to the same direction over the horizon. Noon to noon is most commonly
used, i.e. the time between successive passages due south. However, as measured
relative to the distant stars (the same standard for measuring a year) the “sidereal
day” is closer to 23 hours 56 minutes. This means that a given star will rise about
4 minutes later each day – taking a year for the full range of constellations to
wheel through the night sky.
Next month we’ll discuss calendars.
Lunar phases for January: First Quarter on the 6th, at 1:56 pm; Full Moon at 4:48
am, on the 14th; Last Quarter on the 22nd, at 10:14 am; New Moon on the 29th, at
Early morning sky watchers will see Jupiter bright above the south-southeast horizon
at sunrise, about 37 degrees above the horizon, having risen about two and a half
hours earlier. Mercury is even lower to the east, probably lost in clutter and haze
on the horizon. To the west, Saturn is setting at about 20 degrees above the horizon.
It rose about two hours after sunset, and was visible for most of the night. Venus
returns to the predawn sky at midmonth, as Mercury disappears into the solar glare.
By month’s end it will be about 20 degrees above the southeast horizon at sunrise.
At sunset, Mars begins the month high to the east, about halfway to zenith. Venus
is still very bright to the southwest, but as noted above, it will move into the
predawn sky at midmonth. That means it will be rapidly closing on the Sun’s apparent
position for the first two weeks of January.
At midmonth, about 3 hours after sunset, our overhead view finds the sky divided
by the Milky Way, running southeast to northwest. The constellation Perseus, which
really doesn’t have any extremely bright stars, is at zenith. If we follow the Milky
Way to the northeast, we’ll find Cassiopeia looking very much like a crooked M,
then Cygnus very low on the horizon, looking very much the “northern cross.”
We have better luck finding bright stars and constellations to the east and southeast.
Capella and Elnath are the first bright stars as we look down toward the east. They
are the two brightest stars in the constellation Auriga, about 20 degrees from zenith.
More towards the southeast, we find Aldebaran, marking the apex of the V shape of
Taurus. This lies above the familiar shape of Orion, with the bright red giant Betelgeuse
to the upper left, and bright blue giant Rigel to the lower right. The three stars
marking the “belt” are, from left to right, Alnitak, Alnilam, and Mintaka. (By the
way, we note from the large number of stellar proper names beginning with “Al” or
“El” that most of these names come to us from Arabic.) The brilliant blue-white
star below Orion is Sirius. About 40 degrees above the eastern horizon we find the
bright twins Castor and Pollux, and just beginning to rise is the constellation
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.