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Sky Report -July 2009

Sun Farthest, Jupiter into evening sky and Striking Sights at Dawn

Early on July 3rd, the Earth is farthest from the sun for the year at a distance of 94.5 million miles. Our summers are due to the tilt of the Earth’s axis and the sluggishness of the Earth in responding to the increased sunlight of summer days.

In July, the very bright planet Jupiter slowly creeps into the evening sky as its rising time drops from 12:15 a.m. at the start of July to 10:15 p.m. as the month ends. Jupiter appears as a steadily shining point, brighter than any night star.

In the 5 a.m. dawn, the crescent moon can be seen near two planets, a bright star and the 7 Sisters star cluster on July 17th, 18th and 19th. On the 17th, the moon is above and to the right of the planets  Venus, Mars,  Aldebaran (Bull’s eye star) and the 7 Sisters star  cluster. On the 18th, the moon appears near the planet Mars and the 7 Sisters.  On the 19th, the moon is to the left of the brilliant planet Venus. If any of these three dawns is clear, you will be rewarded with a beautiful sight.

Each year, the Earth's elliptical orbit takes us closest to the sun in early January (perihelion) and farthest from the sun in early July (aphelion). The variation in distance is from 91.4 million miles when closest to 94.5 million miles when farthest.  This change of 3.4% in the Earth-sun distance is dwarfed by the amount of sunlight variation and the concentration of sunlight on the ground.

At the start of summer, the energy per square foot throughout the day is about three times as much as at the start of winter. Then the Earth takes about two months to fully cool down, so February usually is the coldest month. In the same way, the Earth takes about two months to fully warm up, so August usually is the hottest month.

As we travel around the sun, the direction we face in the evening sky slowly changes. In a month, the Earth travels about 30 degrees (360 degrees/12 months) along its orbit so the direction of the evening stars  change by 30 degrees. As the Earth rotates all the way around in 24 hours, the direction of  the stars  changes by 15 degrees each hour (360 degrees/24 hours) through the night.

As the Earth both spins and orbits in the same direction, two hours of rotation (15 degrees/hour x 2 hours) produces the same change in direction as one month of orbiting. For this reason, a given star’s rising time is two hours earlier each month. A distant planet such as Jupiter scarcely changes its’ direction relative to the stars in a month, so Jupiter seems to rise about two hours earlier each month. In early July, Jupiter is rising about midnight. At the end of July, Jupiter will be appearing a little after 10 p.m. In mid August, Jupiter will be directly opposite the sun and rising as the sun sets.

The dawn skies of July 2009 display our two closest planet  neighbors, Venus and Mars in the East.. Venus is the brightest point of light in our night sky, being about 100 times brighter than a typical bright star. During July, the planet Mars appears above and to the right of Venus. Mars is now on the far side of its orbit so it appears quite dull compared to Venus.

The crescent or waning moon will join the two planets in the dawn skies of mid July. On the 17th, the crescent moon will look down on the two planets. On the 18th, the crescent moon will be nearest to Mars. On the 19th, the moon will be about the same sky height as brilliant Venus.

Our public planetarium programs on Sundays at Frostburg State Planetarium will resume on September 6th with “White-Blue Marble and Pale Grey Dot”, viewing the Earth and moon as seen from other planets. Call (301) 687-7799 to receive a free Planetarium/Science Center bookmark by leaving your name and address

By Dr. Bob Doyle

To contact Dr. Doyle, his mailing address is Planetarium, Frostburg State University, Frostburg, MD 21532 or by email at rdoyle@frostburg.edu.









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