What’s Up in the Sky – July, 2010
By Peter Burkey
The summertime has its pluses and minuses when it comes to observing the sky. On the plus side are vacations, warm weather and clear nights. On the minus side are mosquitoes, dew, and the fact that it does not get dark until after ten o’clock.
But the good news is that this is one of the best times of the year to view the Milky Way, our home galaxy. Look toward the southern horizon on a dark night at this time of year and you should see a band of light rising up from the horizon, extending overhead, and continuing to the northern horizon. Some mistake it for a band of clouds, but, upon close examination, it is actually a dense concentration of stars. It’s as though we were imbedded inside a blueberry pancake: as we look toward the edge of the pancake, we see lots of blueberries.
But to get a sense of the vastness of our galaxy we need to think in terms of a scale model. If we were to shrink our galaxy by a factor of one billion, the Earth would be the size of a grape, the Sun would be almost five feet in diameter and the most distant planets would be about three miles away. But the nearest stars would still be tens of thousands of miles away, making our model rather too large to visualize.
If we shrink our model down by another factor of a thousand, so that all dimensions are reduced by one trillion, the Sun would be about the size of a mustard seed, and the entire solar system would fit inside a closet. But the nearest star is still on the other side of town, about six miles away.
The space between the stars is extraordinarily empty. Separations between individual stars amount to over a million times their diameters. The stars in our model are like mustard seeds scattered many miles apart. So, when you look at the Milky Way, keep in mind that the stellar universe is a big place, and it is a wonderful accomplishment that, by analyzing the feeble light we collect with our astronomical telescopes, we are able to learn so much about what’s up in the sky.
This month in history:
July 1: Mt Wilson observatory receives 100 in. mirror – 1917
July 4: Supernova, whose remnant is known as the Crab Nebula, is witnessed – 1054
July 6: Newton’s book, Principia is published – 1687
July 11: Skylab reenters atmosphere – 1979
July 20: Humans walk on Moon for the first time – 1969
July 24: First rocket launched from Cape Canaveral – 1950
Here are this month’s viewing highlights:
Planets this month: Venus now getting lower in the west at dusk. Watch nightly as Venus is joined by Mars and Saturn at month’s end. Look for Mercury lower right of planets July 30. Jupiter rises after midnight for most of the month and is high in the SE at dawn.
July 1: Venus, Regulus, Mars, and Saturn in line in west at sunset.
July 4: Last-quarter Moon
July 11: New Moon. Solar eclipse in South Pacific.
July 15: Look for crescent Moon below Mars and Saturn.
July 18: First quarter Moon.
July 25: Full Moon.
“We are a way for the Cosmos to know itself.” – Carl Sagan