Little Mercury puts on a poor show during the first part of September. It barely rises above the western horizon in evening twilight. It sinks even lower each evening and disappears into the solar glare before the end of the month.
At the beginning of September, Venus shines low in the east during morning twilight, appearing lower each morning. By the end of September, Venus has disappeared into the bright solar glare as it swings behind the sun in relation to Earth.
Mars glows faintly, low in the west-southwest at dusk. On the last few evenings in September, Mars passes above the reddish supergiant star Antares.
Enormous Jupiter rises in the eastern predawn sky. It will appear higher in the sky each morning. The volume of Jupiter is so large that over 1,600 Earths could fit inside.
Saturn appears in the west-southwest evening twilight, becoming lower in the sky each day. This beautiful planet has been visited by NASA's Pioneer 11, Voyager 1 and Voyager 2 spacecraft. The Cassini spacecraft, a joint NASA/ESA mission, is currently in orbit around Saturn.
The sun arrives at the Autumnal Equinox on Sept. 22 at 10:29 p.m. EDT. On that date, the hours of daylight and darkness are equal. This marks the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
"With increasing distance, our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial. The search will continue. Not until the empirical resources are exhausted, need we pass on to the dreamy realms of speculation."
- Edwin P. Hubble, The Realm of the Nebulae
Nearly every time you point out an object in the night sky to someone, whether it's a planet, a bright star, or a satellite, the next question usually is "How far away is that?" And I'm sure that same question was on the minds of our ancient ancestors as they gazed upwards into dazzling nighttime skies that had no artificial light pollution. Their ideas of distance were based on how far they walked to find food or to hunt. They had absolutely no idea how large those heavenly objects really were and no conception of the tremendous distances involved in space.
Even today, it's extremely difficult and perhaps impossible for the human mind to fully comprehend the immensity of outer space. In fact, humans themselves are mostly empty space because the distance between the atoms that make up our bodies is incredibly vast compared to the actual size of the atoms. And the inside of atoms themselves is nearly all empty space.
Distances in space are far too huge to measure with "Earth-sized" units of measure. There are three commonly used measures of distance in astronomy, the astronomical unit, the light-year and the parsec.
One Astronomical Unit (AU) equals 93 million miles, which is the average distance between Earth and the sun. This unit is useful for measuring distances within our solar system. The most distant object we can see in our solar system, without a telescope, is Saturn. At its closest approach, Saturn lies about 746 million miles from Earth (8 AU). When Saturn is positioned on the other side of the sun from Earth, it lies over a billion miles away (11 AU).
Even though a light-year sounds like a unit of time, it is actually a measurement of distance. Although the speed of light is incredibly fast (186,000 miles a second or 700 million miles an hour), it still takes a certain amount of time to travel from one place to another. One light-year is the distance light travels in one year in a vacuum (outer space) and is equal to 5,880,000,000,000 miles. The nearest star to Earth (other than the sun) is a red dwarf called Proxima Centauri at a distance of 4.3 light-years (24.9 trillion miles). The star is much smaller than our sun and is not visible from Earth without a small telescope. It would take a spacecraft traveling at 39,000 mph over 77,000 years to reach that closest star. A commercial jet airplane, flying at a constant 500 mph, would require about 5.8 million years to reach that star. Our home galaxy, the Milky Way, is well over 100,000 light-years in diameter and it contains nearly 400 billion stars. It takes light from a star at one end of our galaxy 20 times longer than all of recorded history to reach the other end. The nearest large galaxy to our Milky Way, Andromeda, lies 2.3 million light-years from Earth. And there are many hundreds of billions of other galaxies out there, each containing hundreds of billions of stars.
An even larger unit of astronomical measurement, used mainly by professional astronomers, is the parsec, which equals 3.26 light-years. Astronomers also use the terms kiloparsec (one thousand parsecs) and megaparsec (one million parsecs) when describing the size of galaxies and the enormous distances between galaxies.
Several months ago, astronomers discovered the most distant object ever detected. Using spectroscopes attached to the Hubble Space Telescope and a 10m Keck Observatory telescope (one of the world's largest ground-based instruments), astronomers determined that the faint infrared light in one of their images was actually from an ancient galaxy 13.1 billion light-years from Earth. As technology continues to improve, humans will peer ever farther into space and further back in time, perhaps someday all the way to the beginning.
Editor's note: This monthly guide to the stars is from the Marshall Martz Memorial Astronomical Association and The Post-Journal. For further information, contact the M.M.M.A.A. at www.martzobservatory.org.