# Selenographic Coordinates

(redirected from Colongitude)

## selenographic coordinates

[sə¦lē·nə¦graf·ik kō′ȯrd·ən·əts]
(astronomy)
A coordinate system for specifying positions on the moon's surface relative to the moon's center, consisting of selenographic latitude and longitude, or of a cartesian coordinate system.

## Selenographic Coordinates

numbers by means of which the positions of points on the surface of the moon are determined. Selenographic latitude and longitude are used as such coordinates.

The latitude of a point is the angular distance of the point from the lunar equator and is measured along the meridian passing through the point. North of the equator, the latitude is positive; south of the equator, it is negative. The north pole here is defined as the pole at which an observer sees the moon to be rotating in a counterclockwise direction.

The longitude of a point is the angle between the plane of the meridian of the point and the plane of the initial meridian. The initial meridian is defined as the meridian whose plane passes through the center of the earth when the libration in longitude (seeLIBRATION OF THE MOON) is equal to zero. Longitudes measured to the east of the initial meridian are regarded as positive; longitudes measured to the west of it are considered negative. It should be noted that this definition of positive and negative does not conform to the general rule established for planeto-graphic coordinates.

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The Sun's selenographic colongitude was 60.2[degrees] and the solar altitude ranged from 10.2[degrees] at the centre of Herodotus to 12.3[degrees] at the centre of Aristarchus at 01:30 UT.
Significant as these observations are, the most detailed and reliable known to the writer are those by Harold Hill (1920-2005), the well known British selenographer who over a period of six decades devoted himself to close visual scrutiny of lunar morphology, in particular to the south polar limb, which he frequently studied at very late stages of illumination, and where he 'witnessed comparable effects under morning illumination on many occasions when southern libration has been favourable--the most recent being on 4.4.92 at colongitude 294[degrees].4 when dim extensions to the limb were measured to be 350 arcseconds from fully lit features.' He noted the effect as very 'atmospheric'.
Mean colongitude 227[degrees].4, selenographic latitude +0[degrees].77.
Another nice feature is a one-click function to calculate the next 24 dates and times of any given colongitude, which describes the position of the terminator.
All his drawings are annotated with date and time and technical data relative to the time of observation, such as seeing conditions, colongitude, selenographical latitude, and libration.
Basically, the mosaics are presented according to lunar phase, but rather than labeling each mosaic with the Moon's age in days, Westfall uses the more precise colongitude value, which increases from 270[degrees] to 360[degrees] during first quarter and then runs from 0[degrees] to 90[degrees] for second quarter.
The Plato 'hook', as it is often described, (2) is not readily observed, since its appearance is critically dependent upon a particular combination of selenographic colongitude, libration and topography.
The interior topography of Cassini A can indeed suggest a double crater under certain lighting conditions, and the attached image (Figure 1) taken by myself on 2008 December 6, when the colongitude value was 18.1[degrees] and seeing conditions were marginal, shows what looks like an interior wall spur that appears to indicate a smaller crater occupying the eastern half of Cassini A.
Jamieson to Wilkins' descriptions, we were able to infer a lunar phase of approximately 60%, with the Moon exceeding the first quarter; a colongitude between 16[degrees]-17[degrees], corresponding to a solar altitude over Cassini A between 16.95[degrees] and 17.80[degrees].
The French psychologist Gustav LeBon observed, "Many men easily do without the truth, but none is strong enough to do without illusions." The lighting conditions that deceived O'Neill and Wilkins occur when the Sun has a lunar colongitude near 127[degrees].
A more precise way to locate the terminator's position is by the Sun's selenographic colongitude, the Sun's position in its monthly circuit around the lunar sky.
Located at lunar coordinates 25[degrees]S 25[degrees]E, Larrieu's Dam is visible under sunrise conditions on a waxing Moon over colongitudes 343[degrees] to 355[degrees] (1.6 to 0.5 days before First Quarter).

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