IAU directions

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IAU Directions

(glossary entry)


The convention that "east" on the Moon is the direction towards Mare Crisium, and "west" is in the direction towards Grimaldi.

Additional Information

Lunar maps were traditionally drawn with the limb that generally faces the observer's eastern horizon labeled "East" and the side towards the Earth's western horizon labeled "West". It was also traditional to draw the maps with "South" (the direction most towards the Earth's south when viewing the Moon with the naked eye) at the top (this was said to more nearly match the view seen in certain kinds of telescopes looked through in a certain way). When such a map is viewed with north up, "East" is on the left and "West" on the right (backwards to maps of places on Earth). Although this tradition seems to date back to William Gilbert, who in around 1600 drew a crude naked eye sketch of the Moon with what me now call Mare Imbrium labeled "Regio Magna Orientalis" (Orientalis being Latin for East) and the modern Mare Serenitatis (+ others) as "Regio Magna Occidentalis" (Occidentalis = West), this has caused considerable confusion. Webb's Handbook (1859+), for example, warned prospective lunar observers that one of the first things they would have to learn was that East and West on lunar maps were reversed from what they might expect.

Since 1961 it has been the recommendation of the IAU that lunar maps intended for exploration be drawn with North up and with the direction towards Mare Crisium regarded as East. This recommendation was made to bring lunar maps into conformity with the more familiar terrestrial maps, in which, when north is at the top, "West" is to the left and "East" to the right. It was apparently felt that having a single and consistent system of maps directions would be less confusing for astronauts and those working on getting men to the Moon, and perhaps avert the kind of disaster that can arise from using mixed unit systems. However this has lead to certain anachronisms, such as Mare Orientale (Latin for the "Eastern Sea") now falling on the Moon's southwest limb. Mare Orientale has not moved, but the system of directions has.

The second part of the IAU recommendation was that lunar charts intended for use at the telescope continue to be printed South-up, but with the East-West terminology entirely omitted. In fact, South-up charts have all but disappeared and the recommendation that the East-West designations be omitted seems to have proved impractical, for observers need a way to describe the locations of features relative to one another. So east-west indications following the "astronautical" convention have been added to nearly all lunar maps, whatever their purpose.

  • More on South up: Until the late 1960s most observers of the Moon used reflectors that showed the Moon, when it was near the meridian, with South at the top of the field of view (assuming they are in the northern hemisphere and standing behind the telescope, looking South); or astronomical refractors (which show the Moon in the same inverted orientation). With the widespread use of Schmidt-Cassesgrain telescopes since the 70s, the commonly used right angle diagonal adds an extra reflection to the lightpath resulting in images that flip the East and West directions. This makes it very difficult to use almost any lunar map at the telescope. Fortunately, online map programs such as the Virtual Moon Atlas and the Lunar Terminator Visualization Tool, can display maps with any orientation, no matter how perverse. And a few lunar maps have been produced for reversed viewing. - tychocrater Jul 4, 2007
  • The Moon and Earth are unique among solar system bodies in that the longitudinal positions are described by stating an angle, up to 180° east or west of a Prime Meridian. All other solar system bodies measure longitude in a single direction on a scale of 0 to 360°. When lunar longitudes are expressed in a +/- system (without specifying "east" or "west"), it is assumed that "+" means a position towards Mare Crisium (the direction currently known as "east") on astronautical charts. This covention did not change in 1961, but prior to that, the positive direction of longitude (towards Mare Crisium) was described as "west". - Jim Mosher
  • Although the IAU has a very definite convention as to the proper use of east-west directions in connection with surface features on lunar maps, it's not entirely clear they have any convention as to the proper usage of the terms "east" and "west" in general; for example in expressions like "east limb" and "west limb". It seems implicit in the deliberations of the Working Group on Cartographic Coordinates and Rotational Elements (the group empowered to set such rules), that when looking at a north-up view of any planetary body, the direction to the viewer's right can always be regarded as "easterly" and the direction to the viewer's left as "westerly". However the meaning of "east" and "west" is never explicitly defined, and their use on a planet can (and often does) conflict with astronomer's more general use of the terms as descriptions of directions on the celestial sphere ("east" being the direction towards the Earth's eastern horizon. For example, in its descriptive text, the Astronomical Almanac (which is as likely as anything on Earth to adhere strictly to IAU conventions) describes a libration that shifts the Moon's center to a positive longitude, as moving the Prime Meridian to the "east" and bringing into view the "west limb". Because they are using directions on the celestial sphere, they mean by this that the Prime Meridian has moved towards Grimaldi, bringing the Mare Crisium side of the Moon into view. Most lunar observers, having embraced the new astronautical mapping conventions, would describe this sequence in exactly the opposite way. Because features on the Grimaldi side of the Moon have west longitudes, they would say that the Prime Meridian has moved "west", and because all the features around Mare Crisium have east longitudes, they would say it's the "East Limb" that's come into view. In a similar context the Astronomical Almanac describes one of the Moon's principal axes of inertia as being located 214.2 arc-sec "west" of the Mean Earth-Moon point. Since they probably mean "west" on the celestial sphere, this seems to mean the axis is offset from the Mean Earth point in the direction towards Mare Crisium. Sometimes, to avoid confusion, the Almanac will explicitly state whether it means "east" and "west" on the celestial sphere (what Ewen Whitaker calls "sky directions") or "east" and "west" on the selenocentric sphere (a sphere centered on the Moon, which yields the directions of a modern astronautical chart). Other times, they don't say. - Jim Mosher
  • Like the Moon, early maps of the Sun were drawn with the limbs marked according to which horizon on Earth they faced (or, equivalently, their directions on the celestial sphere. The convention for solar mapping has never been changed: professional astronomers continue to describe active regions on the Sun as appearing on the "east" limb, rotating across the Sun's face, and disappear over the "west" limb. But this means they are moving from the Earth's east to the Earth's west. And following a tradition dating back to the 1850's, longitudes on the Sun are numbered (as they still are on the Moon) increasingly positive towards the Earth's western horizon (although a full 0 to 360° system is used). Since the 1961 change in the lunar mapping convention, the Sun is the only solar system body for which a north-up map will have "east" on the left. - Jim Mosher
  • For unknown reasons (perhaps because they didn't appear until the 19th century), the other solar system bodies for which early maps were drawn (Mars, Mercury, Venus, etc.) seem always to have been labeled like the modern astronomical charts: that is with "east" to the right when the map is held north up. So no change was required. Unlike the Moon, a 0 to 360° system was used in most cases, and all these numbers are assigned the same direction. That is, the longitudes run either 0 to 360° E (increasing to the right on a north-up map), or 0 to 360° W (increasing to the left on a north-up map). - Jim Mosher
  • The current recommendation of the IAU is that the longitude system of each solar system body be chosen in the way that makes the longitude of the central point (if viewed from a distant fixed point) increase with time. Longitudes on the Moon (increasingly positive in the Mare Crisium direction) violate this principle, causing the numeric longitude of the subsolar point to decrease with time (see colongitude for the remedy -- it increases with time). Positive longitudes on the Sun and Earth also violate the guiding principle, but all three have been retained for historical continuity. - Jim Mosher

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