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Astronomy Question of the Week

# How fast is the Earth moving?

Week 11

Artist's impression of our galaxy, the Milky Way, with its spiral arms

In order to answer this question, we must first make sure we are aware of what is know as the frame of reference. A surfer hardly moves at all, relative to the wave he is on, but seen from the shore he is rushing towards the observer. Cosmic background radiation provides an absolute reference system for speed in space (see the astronomic question from week 10: Where is the coldest point in the Universe?). It is measured coming almost completely uniformly from every direction of space with a temperature of approximately minus 270 degrees Celsius. A movement in a certain direction can be proven and measured by means of a very slight change in this temperature.

In addition, we must bear in mind that the Earth is simultaneously moving in different ways. It rotates around its polar axis, it orbits the Sun, it moves within the Milky Way as a part of the Solar System and it moves through the Universe as part of the Milky Way.

The larger the linear scales observed, the greater the Earth's speed.The turning of the Earth around its axis, the Earth's rotation, means that a point on the equator is travelling at a speed of around 1,670 kilometres per hour, or 464 metres per second. The Earth already has a speed of almost 30 kilometres per second on its path around the Sun.

It becomes even faster when we observe our galaxy, the Milky Way. The Sun is located around 25,000 light years (one light year = about 9.5 trillion kilometres) from the centre of the Milky Way and needs approximately 240 million years for one orbit. The Sun and the Earth, which move with it as part of the Solar System, have a speed of 220 kilometres per second.

On the basis of measurements, scientists have deduced that the various speed components of our Solar System, which have different directions and therefore partly cancel each other out, add up to about 370 kilometres per second. The Milky Way and its neighbouring galaxies form the so-called Local Group. A speed of around 630 kilometres per second has been calculated for this galaxy cluster. The speeds listed here cannot simply be added up to arrive at an overall speed, however, due to the fact that they refer to movements in different directions.

Contact
 Josef Hoell German Aerospace CenterSpace Administration, Space Science Tel.: +49 228 447-381 Fax: +49 228 447-745
Last update: 12/03/2009 12:28:32
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