February 2013 Sky from the Keeble Observatory
Among the regular inclusions for this column are the dates and times for lunar phases.
I’m fairly certain that your calendar at home only includes the dates, and those
may differ by a day from the information below. Let’s see why.
First we have to understand what causes the phases of the Moon. (No, it’s not caused
by Earth’s shadow! Don’t confuse the phases with an eclipse.) Our Moon is in an
elliptical orbit at an average distance of 384,400 kilometers – about 240,000 miles
– or a little less than double the odometer reading on my car! That orbit is tilted
about 28 degrees from the Earth’s equator, or about 5 degrees from the ecliptic,
which represents the plane of the Earth’s orbit around the Sun. The imaginary points
where the orbit crosses the ecliptic are called the nodes of the orbit, and the
imaginary line connecting them is the “line of nodes.” (Don’t you love astronomers’
original use of language?) The line of nodes rotates slowly relative to the sky,
taking 18.6 years to complete one precession cycle. This is the origin of the Saros
Cycle, known since ancient Babylon, which allows us to predict precisely the time
and place of eclipses. The Sun is much farther away, on average about 150 million
kilometers or nearly 400 times the Earth-Moon distance. Conveniently, the Sun is
also about 400 times the diameter of the Moon, so they are very nearly the same
size on the sky.
The Moon’s elongation is the angular distance east of the Sun. When the elongation
is zero we have a New Moon. If this occurs within 1 degree of a node, i.e. if the
line of nodes is essentially along the direction to the Sun, we have a solar eclipse.
When the Moon is 90 degrees east of the Sun we have First Quarter (also known as
quadrature). At 180 degrees the Moon is Full; if this happens within 2½ degrees
of a node we have a lunar eclipse. Note that lunar eclipses are more frequent than
solar eclipses, since the necessary angular alignment is less strict. The other
quadrature is Last Quarter, i.e. 90 degrees west. Since the Sun and Moon are both
moving on the sky, these specific elongations occur not just on a particular day,
but at a particular time. The difference of one day either way on a home calendar
depends in part on where the calendar is produced. A Full Moon seen at 2:00 am in
Ashland will be seen at 11:00 pm the previous date in San Francisco! The times below
are all Eastern Standard Time.
Lunar phases for February: Last Quarter on the 3rd, at 8:56 am;
New Moon on the 10th, at 2:20 am; First Quarter on the 17th,
at 3:31 pm; Full Moon on the 25th, at 3:26 pm.
Predawn planet hunters had better be fans of Saturn, because that’s all you get
to see this month. Early in February the ringed planet will be high to the south.
By month’s end it will be a bit lower and to the southeast. Binoculars or a small
telescope will make the effort worthwhile as you can see the rings for yourself.
Evening twilight is almost as sparse. Mars and Mercury are very low to the west
with Mars above Mercury at the beginning of the month. Mars settles lower and Mercury
moves higher – the 16th is probably the best view of Mercury before it
swings back around the Sun. Venus is too close to the Sun to observe. Jupiter imitates
Saturn, beginning the month to the south and drifting to the southwest. It’s visible
for about half the night.
Near mid-February your overhead view about 3 hours after sunset finds Gemini just
southeast of zenith. The two brightest stars are Castor and Pollux, “the twins”
even though they’re very different stars. Castor is a binary, while Pollux is known
to have at least one planet orbiting. Just northwest of zenith lie another bright
pair, a bit more widely spaced. The brighter is Capella in the constellation Auriga
– it’s another binary. The fainter is called Menkelinan. Turning to look due south,
that very bright star is Sirius – it’s actually the brightest star seen from Earth
(other than our own Sun). Above and more to the southwest it’s impossible to miss
the familiar pattern of Orion. And to the west we see Jupiter, midway between Aldebaran,
the brightest star in Taurus, and the Pleiades. Cassiopeia is to the northwest,
tipped on its side so it looks like the upper case Greek letter sigma. Leo is to
the east, with its brightest star Regulus.