The Free Press, Mankato, MN

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September 1, 2011

Starline: Three bright planets in morning September sky

(Continued)

MANKATO —

The harvest full moon falls September 12. The harvest moon is the full moon closest to the autumn equinox. Occasionally it can fall in October. The full moon rises at sunset and normally the moon rises about 50 minutes later each night. But because of the position of the moon around the equinox, the harvest moon rises only about 30 minutes later each night thus illuminating the fields in the evening for several nights.

Equinox means equal night and we usually say that on the day of the equinox the length of daylight equals the length of nighttime. However that is only approximately true. On my Minnesota Weatherguide calendar the sunrise and sunset times on the autumnal equinox, September 23, are 7:01 a.m. and 7:09 p.m. so day and night aren't exactly equal in length.

In fact inspecting the calendar the times of sunrise and sunset are closest to 12 hours apart on Sept. 26. This discrepancy is not a misprint. It is caused primarily by the effects of refraction. The solar disk you see rising is really the refracted image of the real disk just below the horizon.

The definition of sunrise and sunset also contributes to the discrepancy. Sunrise is when the leading edge of the apparent solar disk crosses the eastern horizon but sunset is when the following edge of the apparent solar disk disappears below the western horizon. Also keep in mind that the amount of refraction and the apparent size of the sun vary. Also only on the ocean do you have the horizon that the calculations refer to. All these effects make sunrise and sunset times approximate.

Just a year ago September 30 the discovery of the first potentially Earth-like planet, Gliese 581g was announced. Because this planet is neither too hot nor too cold for liquid water to exist, it has been called the Goldilocks planet. 

Although some researchers have disputed the existence of this planet, other Earth-like planets have been discovered and it is becoming increasingly likely that many Earth-like planets with life exist.          

         

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