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Guru

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Sunrise and Sunset

09/07/2007 4:26 PM

The following question is posed on a NASA site with respect to The Equation of Time.

"I have a question regarding sunrise and sunset. I realize that the winter solstice on 21Dec is the shortest day of the year. Since this date, the days have been gradually getting longer. Sunset has been getting gradually later as expected, however, sunrise continued to come later until the first week in Jan. My question is: what is the cause of this asymmetrical distribution of daylight between sunrise and sunset?"

The Equation of Time

The (incomplete answer) is give in the above link.

The longest and shortest days of the year are at the times of the summer and winter solstisce's, however the earliest and latetest sunrises and sunsets are several days for the solstices. The Equation of Time does not account for this phenomon.

The question for CR4 contributors is WHY or WHAT accounts for the shift of the times of earliest and latest sunrises and sunsets from the solstices.

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Guru
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#1

Re: Sunrise and Sunset

09/07/2007 4:49 PM

Time inertia.

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Guru
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#2
In reply to #1

Re: Sunrise and Sunset

09/07/2007 5:07 PM

LOL... nice one

Does has thermal inertia too..this is why January is colder than December!

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Power-User

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#3

Re: Sunrise and Sunset

09/08/2007 5:43 AM

I have read the response from the NASA site and it appears to me that he does answer your further question. The two phenomena of elliptical orbit and obliquity combine to move the mean high sun and to shorten/lengthen the day respectively. Therefore we do not have the earliest sunrise on the solstice.

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#4

Re: Sunrise and Sunset

09/08/2007 2:28 PM

I thought the NASA sight gave a reasonable answer. Let me try to restate the issues.

1. The earth rotates on its axis at a constant speed. (I know that it does vary, but remember that a 16 minute variation each year is what we are considering for the equation of time. So the earth's rotation is a constant for the purpose of our discussion.)

2. The earth's axis is tilted with respect to the plane of its orbit around the sun. This obliquity is what gives us the seasons. The earth being closest to the sun on Dec. 21. If the earth moved around the sun in a circle, then Dec 21 would be the date of the latest sunrise, the earliest sunset, and the shortest day.

3. The earth revolves around the sun in an ellipse and it is closest to the sun around Jan 2. That means the earth is moving fastest on Jan 2. As the earth approaches perihelion ( its closest approach to the sun), it is moving faster than its average speed. That means if the sun starts out at noon on day 1, then on day 2 the sun will arrive at noon on our clocks later! In the sketch above, the the dark line represents the earth at noon by our clocks. If the earth were moving at the mean speed, then the black line on day 2 would point to the sun. However, the earth is moving faster than average. That means the earth must rotate to the red line to point to the sun. Hence the sun is overhead after 12 noon. Due to the equation of time, the sun may be 16 minutes late in reaching the meridian.

As a result of this, the shortest day is on Dec 21 due to the obliquity or tilt of the earth's axis of rotation.

The latest sunset is in early December, while the latest sunrise is in January.

Here is another reference to the equation of time.

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