Solar longitude, commonly abbreviated as Ls, is the

ecliptic longitude The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bodi ...

of the sun, i.e. the position of the sun on the

celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...

along the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...

. It is also an effective measure of the position of the earth (or any other sun-orbiting body) in its orbit around the sun, usually taken as zero at the moment of the vernal equinox. Since it is based on how far the earth has moved in its orbit since the equinox, it is a measure of what time of the

tropical year A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time fro ...

(the year of seasons) the planet is in, but without the inaccuracies of a calendar date, which is perturbed by leap years and calendar imperfections. Its independence from a calendar also allows it to be used to tell the time of year on other planets, such as Mars. Solar longitude does not increase linearly with time, the deviation being larger the greater the

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...

of the orbit. For instance, here are the dates for multiples of 90° solar longitude on Mars in the mid 1950s:

Of meteor showers

Solar longitude is especially used in the field of

meteor shower A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at extre ...

s, because a particular meteor shower is caused by a stream of small particles very close to the elliptical orbit of a

comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...

, or former comet. This means that the shower occurs when the earth reaches a particular point in its own orbit, designated by the solar longitude. The value of the solar longitude, like any ecliptic longitude, depends on the

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...

being used. The solar longitude for a given meteor shower would therefore not be constant if the current date were used as the epoch. For this reason, a standard epoch is used, usually J2000.

Of Mars

The Martian year can be divided into 12 Martian months of unequal duration, with the breakpoints being at solar longitudes that are multiples of 30°.

References

{{Portal bar, Physics, Astronomy, Stars, Spaceflight, Outer space, Solar System Astronomy Astronomical coordinate systems

Of meteor showers

Of Mars

See also

References