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Sky Report -August 2009

Jupiter Closest & Dusk Planet Line Up As Daily Sunlight Starts to Drop

The planet Jupiter is closest to the Earth in mid August, rising about sunset and hanging in the sky all through the night. Jupiter appears as a bright steady point low in the Southeast, outshining all night stars.  There are  two other planets that can be seen low in  the western dusk, the inner planet Mercury and the giant planet Saturn. Mercury moves about the sun every 3 Earth months; so six  times a year, Mercury shines low  in either the western dusk or the eastern dawn. Through most of August, Mercury can be glimpsed about 40 minutes after sunset close to the western horizon. On August 17th, the two planets Mercury and Saturn appear about three degrees (about 6 moon widths) apart. Mercury is on the left and slightly brighter than distant Saturn. In August, we lose more than an hour of daily sunlight as the duration of sunlight shrinks from 14.25 hours in early August to about 13 hours by the end of the month.

Each year the giant planet Jupiter appears opposite the sun, then appearing closest to the Earth. This year, Jupiter is in this position (called opposition) on the evening of August 14th. Jupiter will then be about 374  million miles from the Earth. Light then reaching us from Jupiter’s cloud tops will have taken 2009 seconds or 33.5 minutes to reach us.  Jupiter has four large moons that are  easily seen with a small telescope. Also visible will be a faint pair of cloud belts on Jupiter’s disk. Jupiter will stay in evening view through the holidays.

The other two planets in evening view this August are Mercury and Saturn. Depending on the positions of the inner planets on their orbits, Mercury occasionally is the closest planet to the Earth; this is the situation this August as both Mercury and Earth are both on the same side of the sun. Mercury lies to the East of the sun so it can be seen low in the western dusk. In early August, Mercury sets an hour after the sun; by late August, Mercury’s angle to the sun will be smaller and the innermost planet will be setting only 35 minutes after sunset. Also in the western dusk is the giant planet Saturn. Saturn is about 25 times as big as Mercury but is also about 24 times as far from the sun. As a consequence, Saturn appears slightly dimmer than Mercury as seen from the Earth. On the evening of August 17th, the two planets will line up as seen from the Earth with the dimmer Saturn appearing further to the right. Find a place with a flat western horizon and look around 8:40 p.m.; binoculars will help in viewing the two planets. In September, both planets will have moved to the other side of the sun and will begin to be seen at dawn.

In early August, the sun is rising about 6:15 a.m. and setting about 8:30 p.m. (Actual times of both sunrise and sunset depend on your horizon. If you live in a valley with a big hill in the East, you may lose an hour of sunlight in the morning. Or if there is a big hill in the West from your location, the sun could be disappearing an hour earlier than above.) By the end of August, the sunrise is about 6:40 a.m. with the sunset about 7:50 p.m. So in August, we lose about an hour of daily sunlight. September loses about an hour and a quarter of daily sunlight. October has about the same loss. In November, the sunlight loss during the month drops to less than an hour. On the 2nd Sunday of November, we shift to standard time, but this only affects our clocks, not the duration of sunlight.

Public Programs at the Frostburg State Planetarium resume on September 6th with “Blue-White Marble and Pale Grey Dot”, a view of the Earth and moon as seen from other planets. To get a free planetarium schedule/bookmark, call (301) 687-7799 and leave your name and mailing address.

By Dr. Bob Doyle

You can contact Dr. Doyle by mail at

Frostburg State University
Frostburg, MD 21532

or by email at rdoyle@frostburg.edu.









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