Solar longitude, commonly abbreviated as L_s (pronounced ''ell sub ess''), is the

longitude Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...

of the Sun as seen from a given body, 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 orbital plane of that body. 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 – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronom ...

(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-Centre (geometry), center, in geometry * Eccentricity (g ...

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 ext ...

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 warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...

, or former comet. This means that the shower occurs when Earth reaches a particular point in its own orbit, designated by the solar longitude. For example, after passing the

March equinox The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the ver ...

, the solar longitude (λ_☉) of the April

Lyrids The April Lyrids are a meteor shower lasting from about April 15 to April 29 each year. The Radiant (meteor shower), radiant of the meteor shower is located near the constellations Lyra and Hercules (constellation), Hercules, near the bright sta ...

is 32°. 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 b ...

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