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USPS Star Calendar for 1-7 September 25 August 2013

Posted by amedalen in September 2013.
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1 Sep    In the east before dawn, the waning crescent moon stands between the Gemini Twins, 1 fist-width to the upper left, and Procyon, the same distance to the lower right. Brilliant magnitude –2.0 Jupiter is 1 fist-width directly above the moon. Less than 20 percent of the moon’s surface is illuminated. The equation of time is zero.

2 Sep    Mars is 3 finger-widths to the slivered moon’s upper left before dawn.

3 Sep    Only two days from new, the moon, about 5 percent illuminated, rises less than two hours before the sun.

5 Sep    Low in the west at dusk, the first light you see is magnitude –4.0 Venus, followed by magnitude 0.7 Saturn, 1 fist-width to the upper left, and magnitude 1.2 Spica, less than 1 finger-width to Venus’ lower left. New moon at 1136 UT

7 Sep    Only a few days old, the moon sets soon after the sun, making stargazing easier. Ursa Major, the Big Dipper, sits a little left of north with its handle pointing to the upper left. The pointer stars at the end of the bucket point toward Polaris, less than 3 fist-widths to the upper right. Looking to the right (east), you can easily make out Cassiopeia, the Lazy W constellation. Turning farther right, now facing south, you can see the Summer Triangle to Cassiopeia’s upper right. Sagittarius and Scorpius are easy to spot near the horizon. Finishing the turn, now facing west, you can see Arcturus, the bright star in the middle of the sky.


1. George Prescott - 25 August 2013

For Sept. 1 you mentioned equation of time. Please refresh my recollection of the concept.

amedalen - 26 August 2013

The equation of time is the difference between Apparent Time (sundial time), and Mean Time (clock time). Navigators use it to adjust Mean Time to Apparent time. The sign is positive (+) if the sun is running ahead of clock time and negative (-) if the sun is running behind clock time. It is never more than 16 minutes, 4 seconds.
Now, the technical stuff: According to Kepler’s second law, the speed of the earth in its elliptical orbit around the sun varies with the distance from the sun. The earth moves faster as it approaches the sun at perihelion, and slower as it moves away at aphelion. So, from our point of view the sun appears to move faster in January than it does in July. In addition, the 23.5 degree tilt of the earth causes the hour angle of the sun to change at a variable rate as observed from here on earth.
Want more? See Bowditch chapter 18.
Arnold Medalen

George Prescott - 27 August 2013

Dear Arnold, thanks for your response. I’ll check Bowditch. Can you offer me a practical application?

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