Jimmy Westlake: Look out for that harvest moon
September 21, 2010
Steamboat Springs — There's a lot going on in the skies over Colorado this week. Here's a rundown of the week's big events:
Two giant planets, Jupiter and Uranus, reach their closest point to the Earth for the year today. Look for dazzling Jupiter rising in the east as darkness falls. You'll need binoculars to see fainter Uranus, about three-quarters of a degree above Jupiter and nearly lost in its glare.
At 9:09 p.m. Wednesday the sun crosses the celestial equator on its way south for six months. This marks the moment of the autumnal equinox in the northern hemisphere, but for the southern hemisphere, it marks the spring equinox. We now are rapidly exchanging minutes of daylight for minutes of darkness each day as we move toward the December solstice. So long summer, hello fall!
Traditionally, the first full moon of fall is called the harvest moon, and this year, the moon is full less than 24 hours after the equinox. Watch for that big old harvest moon to rise over the eastern mountains just as the sun sinks below the western mountains Wednesday and Thursday nights. As an added bonus, the harvest moon will appear very close to Jupiter on both nights.
If you are the least bit observant, you'll notice something unusual for several days surrounding this full moon. Typically, the moon rises about one hour later each night, but, at the time of the harvest moon, it rises only about 25 minutes later each night for several nights in a row. This means that a big, bright full moon replaces the sun and provides a little extra light as darkness falls. Farmers in particular welcomed the extended hours of light right at the peak time of harvesting the fields, hence the popular name for this month's full moon. The effect is even more pronounced the farther north you go. In fact, up around the latitude of Anchorage, Alaska, the harvest moon actually can rise earlier the second night. That's just weird. It all has to do with the 23.5 degree tilt of the Earth's axis and the 5 degree inclination of the moon's orbit.
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Some folks are convinced that the harvest moon looks much bigger than other full moons. When seen near the horizon, the rising full moon can appear abnormally large in size. How big does it look to you? As big as a pumpkin? Believe it or not, you can cover that giant harvest moon with the tip of your pinky finger held at arm's length. The moon's bloated appearance when seen near the horizon is a famous optical illusion called the "moon illusion." It is really no larger when seen near the horizon than when seen overhead. There's something about holding up your pinky finger that shrinks that big full moon down to size.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all across the world. His "Celestial News" column appears weekly in the Steamboat Today newspaper and his "Cosmic Moment" radio spots can be heard on local radio station KFMU. Check out Jimmy's astrophotography website at http://www.jwestlake.com.