Size of a solar eclipse
The size (also magnitude ) of a solar eclipse is a measure of the portion of the sun's diameter covered by the moon at a certain point and time. With a partial or ring-shaped eclipse this value is less than 1, with a total eclipse greater than 1. During a solar eclipse, the size of the eclipse slowly increases in one place, reaches a maximum value and then decreases again. The maximum achievable size for an observer at a suitable location is an important parameter and can be found as the “size of the darkness” in relevant tables.
In addition to the usual definition used by NASA , there is another definition for size from the Bureau des Longitudes in Paris, which provides matching sizes for the partial phase, but not during a ring-shaped and total eclipse.
Another way to describe the extent of the eclipse is the degree of coverage , which expresses the proportion of the area of the solar disk covered by the moon and is specified as a percentage (see solar eclipse ).
Partial eclipse
The size of a partial solar eclipse is the ratio between the part of the sun's diameter covered by the moon and the entire apparent diameter of the sun . Since the moon does not completely cover the sun during the partial phase, the value is less than 1.
Total or annular eclipse
According to the usual definition, the size of a total or annular solar eclipse is the ratio between the apparent diameters of the moon and the sun. The value is greater than 1 for a total eclipse and less than 1 for an annular eclipse.
However, this definition leads to a discontinuity in the course of the size during the transition from the partial to the total phase: at the moment when totality begins, the size reaches the value 1.0 and then jumps to 1.05, for example.
Alternative definition by the Bureau des Longitudes
According to the definition of the Bureau des Longitudes , the size corresponds to the length of the distance that runs through the center points of the sun and moon and extends from the edge of the moon closest to the center of the sun to the edge of the sun closest to the center of the moon, expressed as the ratio to the sun diameter (see figure). This definition provides the same numerical values during the partial phase as the first definition, but avoids the discontinuity at the beginning of the totality.
The size according to this definition also provides a statement about how centrally the moon and sun disks are on top of each other. The maximum value is greater, the more central moon and sun disks are on top of each other.
Comparison of both definitions
Both definitions lead to the same numerical value during the partial phase, but provide different values for the central phase. Since the standard definition takes into account the full diameter of the lunar disc in the case of a total eclipse, while the alternative only takes one of the two protruding edges into account, the size of the eclipse is smaller according to the alternative definition than according to the usual one. With a ring-shaped eclipse it is the other way round. This must be taken into account when comparing information from different sources. NASA uses the standard definition for its eclipse tables , the French IMCCE (Institut de Mécanique Céléste et de Calcul des Ephémérides) the alternative.
| June 1, 2011 partially |
January 15, 2010 annular |
July 22, 2009 total |
||
|---|---|---|---|---|
| NASA (standard definition) | 0.6010 | 0.9190 | 1.0799 | |
| IMCCE (alternative definition) | 0.6014 | 0.9599 | 1.0404 |
In the case of partial eclipse, both statements agree (the slight difference is due to the ephemeris and astronomical constants used). In the case of the other two eclipses, the information differs in the sense mentioned. According to the alternative definition, the difference between the maximum value and 1 is only half as large as the difference between the constant value and 1 with the standard definition.
Individual evidence
- ↑ a b c d e J. Meeus: Mathematical Astronomy Morsels III . Willmann-Bell, Richmond 2004, ISBN 0-943396-81-6 , chap. 20th
- ^ Fred Espenak : Key to Catalog of Solar Eclipses . Retrieved March 14, 2008: “Eclipse magnitude is the fraction of the Sun's diameter obscured by the Moon. For annular, total and hybrid eclipses, this value is actually the diameter ratio of Moon / Sun. "
- ↑ Les différentes phases d'une éclipse de soleil . ( Memento of the original from November 28, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. IMCCE, accessed January 24, 2008: “À un instant donné la grandeur g de l'éclipse est l'inverse du rapport du diamètre du Soleil sur la distance du bord du Soleil le plus rapproché du center de la Lune au bord de la Lune le plus rapproché du center du Soleil. "
- ^ F. Espenak: Five Millennium Catalog of Solar Eclipses: 2001 to 2100 . Retrieved March 14, 2008
- ↑ Éclipses de soleil et de lune between 2000 and 2050 . IMCCE, accessed January 24, 2008
