why do we have sunrise and sunset

At
the fall equinox, the Sun rises due east and sets due west. It
continues on it's journey southward until,
at the winter solstice, the Sun rises are far to the south as it ever does,
and sets as far to the southwest. Many, if not most, prehistoric cultures tracked these
rising and settings points with great detail. If they had jagged mountains along the horizon, the exact points could be
readily remembered. Without a suitably interesting horizon, standing stones
could be arranged to line up with
the various rising and setting points. Or, tree poles could replace
the standing stones. Or, rock cairns could be used.
This means that the Sun does not rise earlier by a fixed number of minutes every morning and set later by the same number of minutes.


A quick glance at an astronomical almanac or a calendar showing sunrise and sunset times reveals that these do not change evenly. While the Sun rises only a little earlier every day at the beginning of the year, it sets noticeably later every evening. For example, on February 1, 2009 in Berlin, the Sun rose 25 minutes earlier than on January 1, but it set 45 minutes later. In spite of this 'asymmetrical' change, the 'solar noon' the moment when the Sun appears highest above the horizon always falls midway between sunrise and sunset.


The amount of time between sunrise and solar noon remains more or less the same as the amount of time between solar noon and sunset. Thus, the days get longer but the Sun actually only rises a little bit earlier every day and still reaches its highest point in the sky at midday. This means that the Sun reaches this highest point with a 'delay' that increases with every day. How can this happen? For one thing, the Earth does not orbit the Sun at a constant speed, as its orbit is slightly elliptical rather than perfectly circular. This means that the Sun passes through the sky at a slightly different speed from day to day, sometimes a bit faster, sometimes a bit slower.


For another, the Sun's path across the sky is slightly different every day, depending on the time of the year. For another, the Sun's path across the sky is slightly different every day, depending on the time of the year. The reason for this is that the Earth's axis of rotation, which connects the geographic North and South Poles, is not perpendicular to its orbit: depending on the time of the year, either the northern or the southern hemisphere is tilted towards the Sun. German Aerospace Center

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