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Archive Astronomy Questions of the Week - International Year of Astronomy 2009

Why do the times at which the Sun rises and sets not change evenly?

Week 16

 Movement of the Earth around the Sun
zum Bild Movement of the Earth around the Sun

The winter solstice, which occurs on 21 or 22 December, marks the shortest day and the longest night of the year. Following the winter solstice, the days – or, more exactly, the period between sunrise and sunset – get longer again. At first you hardly notice it, but it gradually becomes more and more obvious. 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 1 February 2009 in Berlin, the Sun rose 25 minutes earlier than on 1 January but it set 45 minutes later.

The Sun is 'delayed'?

 The equation of time
zum Bild The equation of time

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.

Dr. Manfred Gaida
German Aerospace Center

Space Administration
, Space Science
Tel.: +49 228 447-417

Fax: +49 228 447-745

Last update: 20/04/2009 09:54:26