Darkness limit
The eclipse limit is the limit of the area within which a solar eclipse or a lunar eclipse can take place. These limits can be expressed in relation to the lunar node as the ecliptical angle Δλ between the moon and the lunar node . One speaks here of node distances .
Different eclipse limits apply to different types of eclipse events. The limits are different for solar eclipses than for lunar eclipses, and further differentiated according to the shape for a total, partial or penumbral eclipse. If the moon is east of the node, the values are positive (+), if the moon is west of the lunar node, negative (-).
Eclipse limits for solar eclipses
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The point of intersection between the ecliptic plane and the ascending or descending lunar orbit , viewed from the center of the earth or from the sub-solar point , is called the lunar node . If the moon is exactly in the node at the new moon , a central (central) solar eclipse ( gamma = 0) takes place. The sun can either be completely covered or in a ring .
If the new moon is not in the node (gamma ≠ 0), the sun can only be partially or not at all eclipsed from the subsolar point. However, the eclipse can still be observed as a central one (and as such total or ring-shaped) on earth if the observer is further north or south of the sub-solar point - if the new moon, viewed from there, is then in the "local lunar node" is located. With a value of Gamma = ± 1, the observation site is shifted towards the North Pole or South Pole to such an extent that the eclipse limit for central eclipses is reached.
In the polar regions, up to a value of gamma ≈ ± 1.55, non-central partial solar eclipses take place before the corresponding eclipse limit is also reached for these.
The graphic above shows that an eclipse cycle - here the semester cycle - has its limits within the two limits for partial eclipses. The stated values Δλ = ± 10.6 ° and Δλ = ± 16.6 ° have been determined with the mean values of the distances earth-sun and earth-moon and for the mean angle between the ecliptic and the lunar orbit.
Eclipse limits for lunar eclipses
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If the full moon is in the node of the lunar orbit, then a central (central) lunar eclipse takes place. If the full moon is not exactly in the node, the earth's shadow does not hit it in the middle. Because of the relatively large earth, the moon can still be hit by its umbra .
The limit for a total lunar eclipse is reached when the moon touches the edge of the umbra from within.
The limit for a partial lunar eclipse is exceeded when the full moon emerges completely from the umbra of the earth.
At this moment the edge of is mostly already penumbra crossed, and thus it is only a partial penumbra -Mondfinsternis instead. Their limit is then reached when leaving the penumbra. A so-called total penumbral lunar eclipse is rare, because the width of the penumbral ring is in most cases smaller than the lunar diameter.
The values Δλ = ± 4.7 °, Δλ = ± 10.6 ° and Δλ = ± 16.7 ° given in the above graphic apply to the mean values of the earth-sun or earth-moon distance and the mean angle between ecliptic and lunar orbit plane .
The numerical similarity of different values for solar and lunar eclipses results from different relationships and is therefore random.