1999 July Sky from the Keeble Observatory
Just how big is it? A casual look at the night sky gives little hint to the scale of our Universe. Assuming we can avoid clouds and city lights, what is it we are likely to see? The nearest non-atmospheric objects we will see, moving points of light crossing the sky in a matter of minutes, are man-made. These Low-Earth-Orbit (LEO) satellites are only a few hundred kilometers above the surface. Just barely beyond the drag of the atmosphere, these artificial moons range in size from a few meters to a few tens of meters. Geostationary communications satellites are some 37 thousand kilometers away, but these can only be seen if sunlight reflects by chance from their solar panels into your binoculars or small telescope. The Moon, of course, is not a human construction. Rather it is a large ball of rock, roughly 1/4 the size of our Earth. It's about 400 thousand kilometers away. Not in the night sky, but certainly the dominant object in our sky, is the Sun. 400 times further away (about 150 million kilometers) it is also about 400 times larger than the Moon. This accident of geometry is what permits the Moon to precisely block the bright face of the Sun during a total solar eclipse.
Let's switch distance scales to one that will serve us better as we include more and more distant objects in our survey. Instead of conventional distances in meters or kilometers, let's measure distance by the time it takes light to travel to us through the vacuum of space. (The speed of light in a vacuum - 300,000 kilometers per second - is a fundamental constant of nature.) Light would travel the equivalent of 7 times around Earth's equator in just one second, taking a negligible fraction of a second to reach those LEO satellites. The geosynchronous satellites are a different story - they're far enough away that nearly an eighth of a second is required for light (or radio) signals to get there from here. The Moon is about a second away; the Sun is eight minutes light travel from Earth.
The next closest star is almost 4 light years distant! Most of the stars you can see on a clear night are within a hundred or so light years. The most distant object you can see with the naked eye is the Andromeda Galaxy (back in our skies this fall) at just over 2 million light years. The most distant galaxy yet detected, discovered with the Hubble Space Telescope and announced just last April, lies nearly 13 billion light years away! Next month, we'll extend the scale just a bit more, back to the faintest and most distant "light" which can reach our instruments on Earth.
Lunar phases for July: Last Quarter on the 6th at 7:57 am; New Moon on the 12th at 10:24 pm; First Quarter on the 20th at 5:00 am; Full Moon on the 28th at 7:25 am.
We're not going to see Mercury at all this month. It reaches inferior conjunction (between us an the Sun) on the 26th. Venus is brilliant in the evening sky to the west. Watching it with binoculars or a small telescope through the month will permit you to observe its changing crescent phase. It spends much of the month in the vicinity of Regulus, in Leo. On the 15th Venus and Regulus will form a bright, compact triangle with a young crescent Moon. Mars begins the evenings to the south, setting in the west about midnight. Don't confuse Mars with the other bright red "star" in the southern sky, Antares in Scorpio. Mars is the one on the right! Jupiter and Saturn can be seen in the early morning just before sunrise. They're to the southeast.
An overhead view at about 10:00 finds the zenith almost devoid of bright stars - Hercules is the constellation most nearly above you, but it lacks really bright stars. With binoculars you may be able to pick out M13, a giant globular cluster in Hercules. On a clear, moonless night, it should be just a bit east of zenith. The southwest quadrant of the sky is marked by the triangle of Arcturus, Spica, and Antares. To the east, we see another bright triangle, consisting of Vega, Deneb, and Altair. The Milky Way runs from north to south, rising about 45 degrees above the western horizon at 10:00.