The Sun: theory



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The time-distance between Sun and Earth

Because the speed of light is finite (299.792,458 km/s), it takes some time before the light of the sun reaches the earth. Moreover, as the spatial distance between the sun and the earth varies throughout the year, the time-distance will vary accordingly.

The graph underneath shows this variation for the year 2005. The right axis shows the distance between the center of the sun and that from the earth in million kilometers. The gravitational influences of the moon and the major planets were taken into account. The distance varies from 147,100 million km (02 Jan 05) to 152,102 million km (05 Jul 05).

The left axis shows the time necessary for the light of the solar photosphere to reach the earth's surface. This means that the above distance was corrected for the radii of the sun (696.000 km) and the earth (6.378 km). The time-distance varies in harmony with the earth's distance to the sun: it is lowest on 02 Jan 05 (488,33 seconds), and highest on 05 Jul 05 (505,02 seconds). On the average, the light from the sun needs 8 minutes 16,7 seconds to reach earth.

Some slight deviations in the time-distance may still be possible. The earth's globe is not exactly spherical, the sun is a plasma and hence has no well defined boundary. On top of this, convective movements in the photosphere due to oscillations, pulsations,... may change the radius of the sun. Nonetheless, it concerns only small deviations in the order of a few hundred kilometers maximum. Thus, their influence is negligible and the calculated time-distance is correct within at least one hundreth of a second.

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