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The Big Bang

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# 6.2.4 The speed and direction of the Earth's motion

The first significant claim to have detected the motion of the Earth relative to the ‘frame of isotropic 3 K radiation’ came in 1977 from a group at Berkeley, California. They concluded that the Earth is moving at a speed of (390 ± 60) km s−1, in a direction towards the constellation Leo, relative to a frame in which the 3 K radiation is isotropic. Their conclusion resulted from observations of a variation of intensity with angle of the form predicted by Equation 14, which we have called a 24-hour variation.

Now this motion of the Earth through the 3 K radiation will be the resultant of several component motions:

1. The Earth's velocity about the Sun (of magnitude 30 km s−1):

2. The velocity of the Sun itself about the Galactic centre (currently estimated to be of magnitude 230 km s−1):

3. The velocity of the Galaxy relative to the Local Group;

4. Whatever velocity the Local Group has relative to the ‘frame of isotropic 3 K radiation’.

based on R.A. Muller (1978) in Scientific American, 238, 64–74 ©
based on R.A. Muller (1978) in Scientific American, 238, 64–74
Figure 28 The absolute motion of the Earth. The Earth travels in its orbit round the Sun at 30 km s−1 and is being swept around the centre of the Galaxy at 230 km s−1. Experiment shows that the Earth's net speed through the 3 K radiation is about 400 km s−1. The Earth's net velocity lies in the same plane as its orbit round the Sun and at an angle tilted sharply upwards (northwards) from the plane of the Galaxy. In this diagram, the Earth's net velocity is depicted as a heavy arrow centred on the Sun (pointing upwards and to the right), since the two bodies travel together. Both are being carried by the Galaxy's own motion through the 3 K radiation. In order to account for the Earth's motion with respect to the 3 K radiation, the Galaxy must be travelling at about 600 km s−1 in the direction shown by the coloured arrow centred on the disc of the Galaxy.