June 2013 Sky from the Keeble Observatory
“Roll out those crazy, lazy, hazy days of summer!” “Summer-time, and the living
is easy.” It’s that season of the year. Vacation season. Hurricane season. Vivaldi’s
“Four Seasons.” “Tis the season to be jolly!” Graduation season (which is why this
month’s column is late.) BTW - A shout out to all the high school and college graduates
… especially eleven physics students who graduated from Randolph-Macon this year!
So, just what are these seasons, and what causes them? First we have to distinguish
between “tradition” and science. For many, summer begins with Memorial Day and ends
with Labor Day. For a meteorologist, summer runs from June 1 to August 31. But,
I’m an astronomer, so here’s my story of the seasons.
Earth is a planet orbiting the Sun on an elliptical path at an average distance
of 150 million kilometers. We move a bit faster when closest to the Sun in January,
and a bit slower in July when we’re at our greatest separation. (Technically the
closest approach is called perihelion, the greatest is aphelion.) Fortunately, the
ellipse is not very far from circular, so we’re only about 1% closer in January
than in July.
Note that we’re closest in the northern hemisphere’s winter, so it cannot be the
distance that drives our seasons. If we pay attention, we’ll note that the apparent
path of the Sun through our sky (the ecliptic) over a year is tilted 23 ½ degrees
from the equator. This moves the Sun higher in the northern hemisphere sky in summer,
and lower in winter. In the southern hemisphere the seasons are reversed, since
a low Sun for us means a high one for them.
Sunlight heats the earth’s surface, which then heats the atmosphere, and the amount
of heating depends on how steep or shallow the light strikes that surface. Near
the poles, where sunlight is always at a shallow angle (or even below the horizon
so there’s no “insolation”) the temperatures are below freezing for much of the
year. Near the equator, where sunlight is always coming in at a high angle, the
temperatures are well above freezing all year. For those of us in middle latitudes,
we experience changing temperatures and cycle through the seasons.
The Sun is directly over the equator on two days each year. These days are called
equinoxes, and we have one in March and another in September, typically around the
20th of the month, though this drifts a bit because the year is not exactly
365 days. The March equinox begins astronomical spring in the northern hemisphere,
and the September equinox begins astronomical autumn. The Sun is farthest north
of the equator in June, and farthest south in December. These are called solstices,
and astronomical summer begins at the June solstice in the north while the December
solstice marks the beginning of astronomical winter.
Remember that we’re closest to the Sun and moving fastest in January. We’re farthest
away and moving slowest in August. That’s why our winters in the northern hemisphere
are shorter than our summers!
Lunar phases for June: New Moon on the 8th, at 11:56 am; First
Quarter on the 16th, at 1:24 pm; Full Moon on the 23rd, at
7:32 am, and Last Quarter on the 30th, at 12:54 am.
Pre-dawn planet watchers can sleep in … you’ll have to wait until the end of the
month for Mars to reappear low on the eastern horizon. Evening planet watchers have
it only a little better, as Jupiter is in conjunction with the Sun and essentially
unobservable all month. Mercury and Venus are low on the western horizon at sunset,
and will be hard to pick out of ground clutter and haze, although Venus climbs higher
as June advances. Saturn begins twilight high to the south all month.
Our mid-month view, about 3 hours after sunset - i.e. after midnight – finds the
“great square of Hercules” at zenith. The relatively bright globular cluster M13
(Great Globular Cluster in Hercules) spans almost 1/3 degree, and is a naked-eye
object on a clear and moonless night. Binoculars or a small telescope will give
a better view. This cluster contains several hundred thousand older stars (it may
be 10 billion years old!) and is about 150 light years across. It lies about 24
thousand light years away.
Due east of zenith it’s easy to spot the bright star Vega. Indeed, it’s the brightest
star in the sky at this time. The southern horizon marks the direction toward the
center of our Milky Way Galaxy, about 25 thousand light years from the Sun, in the
direction of the constellation Sagittarius. The actual center is obscured from our
view in visible light, but we know that a massive black hole (about 4 million solar
masses) marks the dynamical center. Radio and X-ray astronomy reveals a gas cloud
known as G2 which appears about to fall into this black hole, and we’re looking
with anticipation at this event over the next several years. Observations will tell
us a lot about the true nature of black holes, and will serve as a test of the General
Theory of Relativity.
Ursa Major lies to the northwest, with the familiar “pointer stars” at the end of
the dipper’s bowl marking the direction to Polaris, the North Star. Following the
arc of the “handle” of the dipper – or the tail of the bear! – will take you to
the west, where you will see the bright star Arcturus. Continue the same arc to
the southwest and you’ll find Saturn, about 25 degrees above the horizon, and above
and to the left of Spica.