Autumnal Equinox to Occur Early Sunday Morning September 23, 2007
This Sunday morning at 3:51 A.M. MDT the Autumnal Equinox will occur, signaling the beginning of Fall across the northern hemisphere, while over the southern hemisphere, it will be the beginning of Spring. Since the Summer Solstice in late June, the Sun’s apparent position in the sky has been gradually shifting southward, and the Equinox marks the the moment that the Sun ’s apparent position will lie directly over the equator.
The word Equinox originated from the Latin word aequinoctium, literally meaning "equal night". It refers to the amount of daytime and nighttime being almost equal across the globe. So, on the first day of Fall, "daytime" in this area will be just over 12 hours and 7 minutes.
Wait a minute. Isn’t it supposed to be almost equal, i.e. about 12 hours of day and night? Where does that extra 7-8 minutes come from?
The extra time comes from essentially 2 sources. Much of it comes from the refraction effect of our atmosphere. Due to the atmosphere, the Sun’s light rays are bent such that, from the viewpoint of an observer on the Earth’s surface, the Sun appears to rise a few minutes before it actually does rise. Likewise, the Sun appears to set a few minutes later than it actually does. Also, the sunrise and sunset time is computed from the moment the first edge of the Sun ’s disk appears, and the moment the last edge of the Sun’s disk disappears, not mid-point to mid point. As a result, it is not until the 26th that we actually see close to 12 hours of daytime and nighttime.
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The word Equinox originated from the Latin word aequinoctium, literally meaning "equal night". It refers to the amount of daytime and nighttime being almost equal across the globe. So, on the first day of Fall, "daytime" in this area will be just over 12 hours and 7 minutes.
Wait a minute. Isn’t it supposed to be almost equal, i.e. about 12 hours of day and night? Where does that extra 7-8 minutes come from?
The extra time comes from essentially 2 sources. Much of it comes from the refraction effect of our atmosphere. Due to the atmosphere, the Sun’s light rays are bent such that, from the viewpoint of an observer on the Earth’s surface, the Sun appears to rise a few minutes before it actually does rise. Likewise, the Sun appears to set a few minutes later than it actually does. Also, the sunrise and sunset time is computed from the moment the first edge of the Sun ’s disk appears, and the moment the last edge of the Sun’s disk disappears, not mid-point to mid point. As a result, it is not until the 26th that we actually see close to 12 hours of daytime and nighttime.
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