Sunrise and sunset

[Roger77]

Can someone please explain why, as the days lengthen after the winter solstice, the time of sunrise barely changes for the first two weeks whereas the time of sunset changes by about 13 minutes?
During the period after the summer solstice the reverse seems to be the case with sunrise changing more rapidly than sunset but perhaps not so markedly.

[Clanad]

This may be difficult to visualize, but, after Dec. 21st, the sunrise should get earlier and sunset later, both extending the amount of daylight. As you point out, while the sun does begin to set later, for several days after the 21st the sunrise also gets later.

This curious phenomenon occurs because we rely on watch time rather than sundial time, and because after the winter solstice, it starts taking longer for the sun to make it from one east-west point in the sky back to that same point.

The Earth takes 23 hours and 56 minutes to spin around on its axis. But the average day is 24 hours long because as the earth is spinning, it is also revolving around the sun. The spin makes the sun appear to move from east to west. The revolution makes the sun appear to move from west to east. So for the sun to make it back to the same east-west point in the sky, the Earth needs to spin a little bit longer than one rotation � for about four more minutes.

Therefore, anything that increases the west to east motion will also increase the length of day (the Earth will need to spin a bit longer for the sun to get back to the same point in the sky). After the solstice, both the tilt of the earth on its axis and the variability in the speed of Earth's revolution around the sun increase the sun's west-to-east motion.

(Contd.)

[Clanad]

(Contd.)

Because of Earth's tilt, as we revolve around the sun, the sun moves from being directly overhead at the Tropic of Cancer (latitude 23.5 degrees north) at our summer solstice to being directly overhead at the Tropic of Capricorn (latitude 23.5 degrees south) at winter solstice.

At the solstices, there is no north-south component of the sun's apparent motion, therefore the west-to-east motion is maximal.

Just after the winter solstice, the Earth, in its elliptical orbit, is closest to the sun. The increased gravitational pull of the sun causes the Earth to revolve faster.

Therefore sun time lags behind watch time, and according to our watches (but not a sundial), the sun rises later. (Thanks to Physical Sciences - University of California, San Diego)

[stewey]

This site does not really explain the situation, as Clanad was doing; but. it is really interesting. Jump around it and see.
http://www.timeanddate.com/

[mibn2cweus]

As previously stated the Earth�s orbit (and the orbits of all planets to varying degrees) is elliptical.

The shortest day (the time the Sun is above the horizon) in northern latitudes corresponds to the Winter Solstice, the time of the year when the Earth�s Northern Hemisphere is tilted furthest away from the Sun. The solstice does not however coincide with the Earth�s closest approach to the Sun (perihelion) two weeks later when it is traveling around the Sun at is greatest velocity. The speed required for a planet to maintain an orbit is inversely proportional to its distance for the source of gravity responsible for pulling it into an orbit.

Because the Earth�s rate of rotation is nearly constant while its speed around the Sun is not we observe the anomaly of variable �local noon� times (the time of day the Sun crosses the meridian) each day. In addition to skewing sunrise/set times this is also the reason why there are about three less days between autumn and spring as there are between spring and autumn.

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[john91]

The word 'solstice' itself means that the sun has reached a point where change is very slow.

solstice
c.1250, from O.Fr. solstice, from L. solstitium "point at which the sun seems to stand still," from sol "sun" (see sol) + pp. stem of sistere "to come to a stop, make stand still" (see assist).

Online Etymology Dictionary, � 2001 Douglas Harper