Elongation (astronomy)
In
When an inferior planet is visible after
Astronomical tables and websites, such as Heavens-Above, forecast when and where the planets reach their next maximum elongations.
Elongation period
Greatest elongations of a planet happen periodically, with a greatest eastern elongation followed by a greatest western elongation, and vice versa. The period depends on the relative
Let T be the
where Te and Tp are Earth's and the planet's years (i.e.
For example,
These values are approximate, because (as mentioned above) the planets do not have perfectly circular,
Of superior planets
All superior planets are most conspicuous at their oppositions because they are near, or at, their closest to Earth and are also above the horizon all night. The variation in magnitude caused by changes in elongation are greater the closer the planet's orbit is to the Earth's. Mars' magnitude in particular changes with elongation: it can be as low as +1.8 when in conjunction near aphelion but at a rare favourable opposition it is as high as −2.9, which translates to seventy-five times brighter than its minimum brightness. As one moves further out, the difference in magnitude that correlates to the difference in elongation gradually falls. At opposition, the brightness of Jupiter from Earth ranges 3.3-fold; whereas that of Uranus – the most distant Solar System body visible to the naked eye – ranges by 1.7 times.
Since asteroids travel in an orbit not much larger than the Earth's, their magnitude can vary greatly depending on elongation. More than a dozen objects in the asteroid belt can be seen with 10×50 binoculars at an average opposition, but of these only Ceres and Vesta are always above the binocular limit of +9.5 when the objects at their worst points in their orbital opposition (smallest elongations).
A quadrature occurs when the position of a body (moon or planet) is such that its elongation is 90° or 270°; i.e. the body-earth-sun angle is 90°.
Of moons of other planets
Sometimes elongation may instead refer to the angular distance of a moon of another planet from its central planet, for instance the angular distance of Io from Jupiter. Here we can also talk about greatest eastern elongation and greatest western elongation. In the case of the moons of Uranus, studies often deal with greatest northern elongation and greatest southern elongation instead, due to the very high inclination of Uranus' axis of rotation.
See also
- Aspects of Venus for greatest elongations of Venus
- Conjunction (astronomy)
References
- ^ Chisholm, Hugh, ed. (1911). . Encyclopædia Britannica. Vol. 9 (11th ed.). Cambridge University Press. p. 298.
External links
- Mercury Chaser's Calculator (Greatest Elongations of Mercury)