Moon diameter
The mean moon diameter is 3476 km, which corresponds to 27.3% of the earth's diameter . However, the figure of the moon deviates by 1 to 2 km from the exact spherical shape.
Apparent moon diameter
The apparent diameter of the moon , as it appears from the earth, is almost exactly 0.5 °, as is that of the solar disk .
Because of the elliptical lunar orbit , the apparent diameter of the earth's satellite varies by +/- 6 percent, namely between the values
- 29 '10 "in the distance from the earth (apogee) and
- 33 '30 " near the earth (perigee).
Within a single month , the fluctuation range is somewhat smaller.
This variation is hardly noticeable when looking at the moon with free eyes, but it is noticeable in some solar eclipses . Such an eclipse can only be total when the new moon is close to its perigee . At a greater distance there is only an annular eclipse.
If, on the other hand, a full moon coincides with the proximity of the earth, one occasionally speaks of a super full moon - in a greatly exaggerated choice of words - because the moon disk then appears somewhat larger and brighter than usual.
In amateur astronomy , the apparent diameter of the moon is an often used measure for estimating small angles in the sky. People like to speak of "full moon widths". For example, the two stars of the Polweiser (α and β of the Big Dipper) are 5.5 ° or 11 full moon latitudes apart, and Alkor, often referred to as "Little Rider", is 11.8 'or 0.4 moon diameter from his main star Mizar (ζ Zeta Ursae Majoris ) removed.
See also
literature
- Patrick Moore et al., Royal Astronomical Society: Atlas of the Solar System , 462 p., Chapter 5 The Moon . Herder-Verlag Freiburg-Basel-Vienna 1985
- Karl Stumpff , H.-H. Vogt: The Fischer Lexicon Astronomy . Revised 8th edition, Fischer Taschenbuch Verlag, Frankfurt / Main 1971