August 2009 Sky from the Keeble Observatory
A friend asked me to explain why the Moon sometimes crosses the sky low to the south,
and sometimes high overhead. We notice a similar phenomenon with the Sun, high overhead
in the summer, low in the winter, but that’s an easily predictable phenomenon from
season to season, while the Moon’s high or low passage doesn’t seem to be as regular.
Since this is something that often puzzles folks, I thought I’d share the answer
First, let’s look at the Sun (actually, you should never look directly at the Sun!).
Relative to the background stars, which you can’t see during the day but should
know they are still there, the Sun’s path is called the ecliptic. It passes through
the familiar (at least in name) constellations of the zodiac. This path is tilted
a little over 23 degrees from the plane of the Earth’s equator. Projected onto the
sky, our equator is known to astronomers as the “celestial equator.” If you can
imagine actually drawing it on the sky, you would see this crossing from east to
west, reaching a point to the south which for Ashland is 53.25 degrees above the
horizon. It is to points along this equator that you direct your satellite dishes
to get television signals, and where all the geosynchronous satellites are in orbit.
The ecliptic crosses the celestial equator at two points, making that 23 degree
angle. These are known as the equinoxes. The Vernal Equinox is where the Sun crosses
the equator heading north on the sky, the Autumnal Equinox when it’s heading south.
At the solstices, summer and winter, the Sun is a full 23 degrees above or below
the celestial equator. Also, if the ecliptic is low in the sky in the daytime, it
is high in the sky at night.
Now, let’s add another tilted circle on the sky, to mark the path of the Moon. This
path roughly follows the ecliptic, but the Moon’s orbit is tilted about 5 degrees
to the ecliptic, so the Moon can get as much as 28 degrees above or below the celestial
equator. This path crosses the ecliptic at points called “nodes,” which move around
the ecliptic from month to month. The Moon’s orbital period is about 29 days, so
depending on whether the Sun is high or low, and depending on the actual lunar phase
(which is determined by how far around the sky it is at a particular time), we can
see it either very high or very low.
The overall pattern of their relative positions on the sky repeats on a roughly
19 year cycle, known as the Saros cycle, which was known at least as early
as ancient Babylon. This cycle is shared by the position of the nodes. When the
Sun and Moon are both at the same node, we get a solar eclipse, like the one just
observed over much of Asia last month. When they’re at opposite nodes, we get a
lunar eclipse. And, knowing the cycle we can predict eclipses rather than being
taken by surprise.
Lunar phases for August: Full Moon on the 5th at 10:55 pm; Last
Quarter on the 13th, at 2:55 pm; New Moon on the 20th, at
6:02 am; First Quarter on the 27th, at 7:42 am.
Predawn planet watchers can look for Venus to rise about 3 hours before the Sun,
trailing Mars which rises an hour earlier. Venus will be about 34 degrees above
the eastern horizon at sunrise, and Mars almost 50 degrees. Mars will continue to
climb higher (rising earlier) as the month goes on, reaching 60 degrees above the
eastern horizon by late August dawn. Venus holds nearly the same relative position
all month. Jupiter will have been visible all night, settling low to the west southwest
at dawn early in the month, and setting an hour before sunrise by the end of the
Early evening planet watchers will find Saturn about 23 degrees above the horizon
to the west southwest at sunset. It will set about two hours later, since the plane
of the ecliptic is making a shallow angle with the horizon. Mercury is about midway
between Saturn and the Sun, so you may see it low to the west at twilight, near
the bright star Regulus. These two planets will continue drifting eastward from
Regulus, and will pass within 3 degrees at midmonth, with Saturn above Mercury.
As noted above, Jupiter will be visible all night, rising to the east about an hour
Our overhead view at midmonth finds the bright star Vega at zenith, marking the
western edge of the Milky Way, which crosses the sky from north-northeast to south-southwest.
To the east of Vega, marking the middle of the Milky Way is the familiar constellation
Cygnus, with bright Deneb marking the “tail of the swan” and the beautiful (but
not extremely bright) binary Albireo marking its head. Albireo is faint, but has
the benefit of almost no truly bright stars in its vicinity. West of Vega we see
the small irregular trapezoid of Hercules. Along the lower edge of this “almost
square” asterism lies the magnificent globular cluster known as M13. You may pick
it out as a faint fuzzy patch without binoculars on a clear, moonless night. With
binoculars or a small telescope, it reveals itself as a swarm of millions of stars
resembling what one student described to me as a dandelion gone to seed.
Below Hercules you will see the bright star Arcturus, about 30 degrees above the
western horizon. Low to the southwest is the constellation Scorpio, marked by the
bright red star Antares. To the northwest you will find the familiar shape of Ursa
Major, better known to non-astronomers as the Big Dipper. The two end stars in the
“bowl” of the dipper draw a line toward Polaris, which lies close to but not exactly
at true north.