Vernal Equinox Conundrum?

New Orleans is at 90 degrees west longitude. On Saturday, 20 April, sunrise was at 7:04 a.m. and sunset at 7:12 p.m. I understand that the 8-minute difference between daylight and darkness is partially due to the time it takes for the sun's disk to slip below the horizon and partially due to atmospheric refraction. If the sun were a point source of light and the earth had no atmosphere, the times should be 7 a.m and 7 p.m. But why 7:04 and 7:12 rather than 6:56 and 7:04? Why the 8-minute discrepancy there?

3 Answers

  • 1 decade ago

    Apparent or Sundial Time is not normally the same as Mean Time, in other words, the Sun doesn't always cross the meridian at noon (1 pm in summer). There are 2 reasons for this:

    1) The location may not be exactly at the longitude on which the time zone is based, for example a location at 95º W would have sunrise and sunset delayed by 20 minutes compared to 90º W.

    2) Due to the tilt of the Earth's axis and the fact that the Earth's orbit is not circular, the Mean Sun (on which Mean Time is based) does not usually coincide with the real Sun in the sky, which gets ahead or behind of its mean position during the year. This is known as the Equation Of Time and can exceed 15 minutes. So even a place on 90º W longitude could see the Sun cross the meridian at 11:45 or 12:15 Mean Time.

    There is an explanation here:

  • 1 decade ago

    Your question throws up some questions and inaccuracies.

    1.The date stated should have been 20th March and not 20th April. 20th March is Saturday and 20th April is Tuesday.

    2. it is not clear whether local time differs from that of time zone. If so, by how much. From the figures local noon appears to be at 1:04 afternoon. Was it that the day light saving formula is adapted (I am totally unaware and confused about this procedure, being in India) ?

    3. If it on Vernal equinox both sunrise & sunset should be spaced 6hours on either side. It is not clear.

    If I consider the above (& 'yes', 'yes' as assumptions), local noon appears to be at 13:04 (1:04 afternoon) pointing to a local time of 4 minutes behind the time zone. It implies that the site where the observation was, is 1 degree (corresponding to 4mts.) behind time zone longitude.

  • 1 decade ago

    New Orleans is not in the middle of its time zone. At one edge of the time zone, the clock time is off from the solar time by about +30 minutes. At the other edge, it is off by about -30 minutes. Only on the correct longitude line does the clock time and the solar time match up.

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