SEPTEMBER  1997

054    SEPTEMBER 7, 1997:   Watch Jupiter's Satellites

The fours Galilean satellites of Jupiter: Io, Europa, Ganymede, and Callisto, are easy to watch through a small telescope, just like their discoverer Galileo did in early 1610. This week about 10 p.m., you can see Jupiter in the south. The seasonal differences will place Jupiter much lower in the sky than Galileo saw it. The week starts off with three moons on one side, and volcanic Io by itself. On Tuesday, Callisto (farthest out) and Ganymede (brighter pair) are on one side of Jove, and widely separated, while Io and Europa are closer to Jupiter on the other side. Europa is the outer of the dimmer pair, and currently our best candidate for extraterrestrial life. As the week progresses, the satellites jumble nearer to the planet so that by Saturday, Europa is again positioned alone while the others are grouped together on the opposite side. Watching these four moons revolve around Jupiter helped cement, in Galileo’s mind, that the planets orbited the sun, a "revolutionary" concept for the early 17th century.

055    SEPTEMBER 14, 1997:   Harvest Moon

The moon is full on Tuesday, September 16th, making it the closest full moon to the Autumnal Equinox, the time when the length of the day and night are equal. You may have already guessed that Tuesday’s full moon is called the Harvest Moon. It is at this time of the year that the moon’s orbital path is closest to being parallel to the eastern horizon. Even though each day the moon moves about 13 degrees in its orbit around earth, its motion does not carry it much farther below the horizon. As a result, Earth’s spin causes the moon to reappear within 20 to 40 minutes of its rising time the previous day, keeping the moon visible in the east for three or four nights just after sundown. Before tractors were equipped with headlights, mid-latitude farmers would take advantage of the natural bright light of these "post" full moons and continue to harvest crops late into the evening hours. This Harvest Moon is brighter than most, because the moon reaches its closest position to Earth, called perigee, only four hours before the moon is full.

056    SEPTEMBER 21, 1997:   Summer Triangle Dominates Fall Sky

If you were an avid Hale-Bopp follower during February and March of this year, then you must remember the comet’s track through the pre-dawn sky just below the three bright stars of the Great Summer Triangle. Although fall begins on Monday, at 7:56 p.m., these three summer stars begin the season, dominating the autumn sky. Right after dark, about 8:30 p.m. face your body south, while looking directly overhead. Flanking the zenith (directly overhead) should be two bright stars, Vega and Deneb. Blue-white Vega is to your right, and the brightest star of the triangle. Deneb is the faintest of the triad. Now lower your head about 1/3rd of the way towards the horizon, while still facing south. Altair, the third star should be straight ahead. Connect them and you’ve got the GST. These stars will still be visible right after dark on the first day of winter; however, you’ll have to look near the western horizon to catch them.

057    SEPTEMBER 28, 1997:   Seeing and Transparency

Over the next month, as the seasons change, a few hot and hazy Indian summer days still lie ahead, as well as the cooler, more invigorating periods which traditionally mark the autumn. Look at the sky on a warmer, more humid night, and you’ll probably see fewer stars, but they will be shining with a steadier light. Compare that with a fresh, brisk, fall evening, and you’ll probably agree that the stars appear brighter, but they twinkle with greater vigor. In astronomy, the motion of the air which causes twinkling is called "seeing," while the clarity of the atmosphere, which allows you to see deeper into space is called "transparency." Unfortunately, the conditions which produce good seeing, i.e., a steady atmosphere, are not conducive to very transparent skies. Under excellent seeing, telescopes can resolve closer to their theoretical limits. When the transparency is excellent, telescopes can see dimmer stars, but atmospheric turbulence limits their ability to separate detail. The seeing and transparency conditions are always perfect in space, which is why astronomers have placed so much emphasis on the Hubble Space Telescope.