August 2008 Sky from the Keeble Observatory
A question I am often asked – indeed, one that I insist my students ask is, “How
do we know that?” So, for the next few months I’d like to explore with you some
of the answers to the question.
Let’s first consider how the sky looks without benefit of telescopes or binoculars.
Our daylight skies are illuminated and the Earth is warmed by the bright Sun, an
apparent circle of brilliance about half a degree wide. At various times of the
month we can also follow the waxing and waning Moon, another half-degree wide circle
on the sky which is far dimmer than the Sun. All other astronomical objects appear
as mere points of light, visible only between evening and pre-dawn twilight. (To
be fair, some of them appear as fuzzy patches like the Orion Nebula or the Andromeda
Galaxy.) Some of those points of light remain in apparently fixed geometric patterns.
These “fixed stars” maintain their relative positions as they cycle through the
annual changes of the seasons. Humans have used the changing suite of visible constellations
to mark planting and harvest, hunting and gathering for millennia. Some of the lights,
however, move relative to the background stars. These are the planets, deriving
their collective label from a Greek word meaning “wanderer.”
The Moon lies some 60 times the Earth’s radius from the center of our home planet
– a little under a quarter of a million miles, on average. How do we know? The answer
lies in simple geometry! Imagine two surveyors observing the Moon simultaneously
from two locations separated by a few miles. They would see the Moon in slightly
different positions relative to the stars around it. Using these slightly different
angles, they could draw a triangle of known baseline, and known directions toward
the Moon. Using basic trigonometry (or simply a ruler laid against their scale drawing)
we determine the distance to the Moon. Once we know its distance, its angular width
on the sky also tells us its actual diameter. Again, it’s all simple geometry. The
distance was sufficiently well known by the time of Isaac Newton that he used it
to help figure out his “law of universal gravitation.”
Next month, we’ll describe how we can use more geometry to figure out the distance
to the Sun and the other planets in our solar system.
Lunar phases for August: New Moon on the 1st, at 6:12 am; First
Quarter on the 8th, at 4:20 pm; Full Moon on the 16th, at
5:16 pm; Last Quarter on the 23rd, at 7:49 pm. And, we will have another
New Moon on the 30th at 3:58 pm. The first New Moon in August will be
accompanied by a total solar eclipse – unfortunately it’s not visible from central
Virginia. If you’re vacationing in the Canadian Arctic or Russia, Siberia, and China
you’ll have a chance to see some of it! The Full Moon this month will also include
a partial lunar eclipse, but it will only be visible in North America from the extreme
eastern part of Newfoundland. The best view will be from Europe and Africa.
Predawn planet watchers can sleep in … Jupiter sets hours before sunrise all month,
and there are no other planets to be seen!
Early evening viewers will see Mars, Saturn, the star Regulus, and Venus lined up
above the western horizon as the sky fades to twilight. Mercury is even closer to
the horizon, probably lost in ground clutter and solar glare. Towards the end of
the month Mars, Mercury, and Venus will all cluster within about 3 degrees, 9-13
degrees above the western horizon at sunset.
About two hours after sunset, our overhead view at mid-month finds the bright star
Vega almost exactly at zenith. Nearby, a suitably clear night (unlikely in a Virginia
August!) will allow you to pick out the Ring Nebula with binoculars. This so-called
planetary nebula has nothing to do with planets. Rather, it shows a faint disk on
the sky through binoculars or a small telescope. It’s the remains of a star which
has ejected its outer layers. The hot remaining core of the star emits ultraviolet
radiation which ionizes the expanding gas cloud to emit visible light.
Vega lies on the edge of the faint luminous band of the Milky Way, easily visible
on a moon-less night if you can get away from city lights. This is an edge-on view
from our vantage point in the disk of our home Galaxy. High to the east-northeast,
in the middle of this swath of stars you’ll see the bright star Deneb, in the constellation
Cygnus, marking the point on the sky towards which the Sun in moving as we follow
our roughly circular path in the plane of the Galaxy. To the south-southeast, about
56 degrees above the horizon, you’ll see Altair, in Aquila. Vega, Deneb, and Altair
mark the three corners of the Summer Triangle.
Below the Milky Way to the east we see the sky largely free of bright stars. What
you can see with a little coaching is a widely space asterism of four modestly bright
stars marking the “great square” of the constellation Pegasus. North-northeast we
see the familiar crooked W shape of Cassiopeia. The “big dipper” of Ursa Major is
toward the northwest.
Arcturus lies to the west, about 30 degrees above the horizon – it will set about
four hours after sunset. Low toward the south-southwest we see the bright red Antares
in Scorpio. Due south, near the bright planet Jupiter lies the “teapot” asterism
in the constellation Sagittarius – in his direction lies the center of our Galaxy,
only 25,000 light years distant, and hidden behind vast clouds of opaque dust.