Selenographic Colongitude

The selenographic colongitude of the sun ( SSC for Sun's selenographic colongitude ) is the position calculated from the prime meridian of the moon in the direction of the moon- west from 0 ° to 360 ° of the intersection of the sunrise terminator with the lunar equator, i.e. the point on the lunar equator, when the sun is just rising. ${\ displaystyle S}$

The importance of the colongitude in lunar observation results from the fact that craters and other structures on the lunar surface appear particularly sharply drawn and with long shadows if they are on the illuminated side near the morning or evening terminator.

Temporal course

In the first quarter is the above. Point of intersection exactly in the middle of the moon , then moves towards the west of the moon as the moon continues to wax , i.e. to the left when viewed from the earth, and

reaches the western edge of the moon with a full moon and thus a colongitude of 90 °.

In the third quarter of the terminator of the sun reached among gangs, the moon center, the terminator of the sunrise is then on the invisible center of the far side of the moon , and the Colongitude is 180 °.

In New Moon finally there is the terminator of the sunrise on the right, the eastern edge of the moon, and the Colongitude 270 °:

1st quarter; SSC = 0 °
SSC = 45 °
Full moon ; SSC = 90 °
SSC = 135 °
3rd quarter; SSC = 180 °
SSC = 225 °
New moon ; SSC = 270 °
SSC = 315 °

calculation

The selenographic longitude of the morning terminator is calculated from the colongitude as ${\ displaystyle \ lambda}$

{\ displaystyle \ lambda = 360 ^ {\ circ} -S}

It is normalized to the range [-180 °, + 180 °], i. H. a colongitude of 87 ° (shortly before the full moon) corresponds to a selenographic longitude of 360 ° - 87 ° = 273 °, normalized -87 ° = 87 ° west longitude. The selenographic longitude of the evening terminator is obtained by adding 180 ° to the selenographic longitude of the morning terminator. ${\ displaystyle \ lambda}$

The Colongitude S is calculated as

{\ displaystyle S = L_ {m} -L_ {S} -90 ^ {\ circ}}

It is

L _{S is} the longitude of the sun in ecliptical coordinates
L _m the mean length of the moon: ${\ displaystyle L_ {m} = Li \, \ cos (L- \ Omega) \, \ tan U- \ lambda}$ ${\ displaystyle L_ {m} = Li \, \ cos (L- \ Omega) \, \ tan U- \ lambda}$
- L is the longitude and U the latitude of the moon in ecliptical coordinates
- i the axis inclination of the moon relative to the ecliptic (= 1.534 °)
- Ω the length of the ascending lunar node : ${\ displaystyle \ Omega = 259 ^ {\ circ} 08'-69629 '' (t-1900.0)}$ ${\ displaystyle \ Omega = 259 ^ {\ circ} 08'-69629 '' (t-1900.0)}$
  - t the year.

literature

Patrick Martinez (ed.): The observer's guide to astronomy . Cambridge University Press, Cambridge 1994, Vol. 1, p. 119