arrow_backTutorials - Basic terms

Basic terms

This tutorial will introduce you to the basic terms used in the sky calendar and enable you to recognize most of the astronomical events in it.

Keywords: Inferior and Superior planets, Conjunction, Opposition, Elongation, Perihelion, Aphelion.

Inferior and Superior planets

Planets inside Earth's orbit (closer to the sun) are called inferior, planets outside Earth's orbit are called superior.

diagram showing the inner and outer planets

Conjunction

A planet is at conjunction when it is in line with the Sun as seen from Earth.

The inferior planets (Mercury and Venus) can form such a line by being either behind the Sun or by being between the Earth and the Sun. When an inferior planet is behind the Sun it is said to be at superior conjunction, when it is between the Earth and the Sun it is at inferior conjunction.

The superior planets outside Earth's orbit can never pass between the Earth and the Sun and can therefor only be at superior conjunction. The word superior is therefor usually not used and the planet is just said to be at conjunction.

illustration of both inferior and superior conjunction

Opposition

A planet is at opposition when it is opposite the Sun in the sky. This is not possible for planets inside Earth's orbit, therefor only superior planets can be at opposition.

When a planet is at opposition it rises at sunset, reach its highest point in the sky at midnight and set at sunrise. It also reaches its closest approach to Earth at this time, but because the planet's orbits are elliptical rather than perfect circles some oppositions will bring a planet closer to the Earth than others.

Oppositions are the perfect time to observe a planet through a telescope because it is up all night and will appear at its largest diameter, although the size will vary because of the eccentricity of the planets' orbits. This is very noticable when observing Mars because its orbit is quite eccentric. The result is that it's distance from the Earth, and it's apparent size varies considerably from one opposition to the next.

diagram showing an opposition

You can observe the effect yourself by using the Solar System view. The opposition on 24. July 2018 is a faverable one, so we will start at that date. 780 days is the average time between Mars oppositions, so if we configure the time control panel to 780 days we can step between different oppositions. If you step backwards a few times you'll see that the distance between Mars and Earth will increase greatly during the oppositions because of the high eccentricity of Mars' orbit.

We can do the same one more time with the same setup, but this time we will open the planet round-up view. Notice how the size of Mars will decreases as you step backward. The angular diameter of Mars will vary from under 14" to over 24" (arcseconds) during opposition.



Elongation

Elongation is the angular distance between the Sun and a planet (or the Moon) as viewed from Earth. In other words the Sun-Earth-Planet(or Moon) angle.

Because the inferior planets orbit inside Earth's orbit their elongation is restricted to a limited range.

Any elongation is possible for the superior planets.

diagram showing maximum eastern and western elongation of an inferior planet

The Maximum possible elongation of Mercury is about 28°, for Venus the greatest elongation is about 47°.

The greatest elongation of a superior planet is 180° when the planet is said to be at opposition. During a conjunction the elongation is 0°.

When the elongation of the Moon is 0° it is said to be a new Moon. When its elongation is 90° it is in first or last quarter and when the elongation is 180° it is full.

You can observe the relationship between the elongation and phase by using the Planetfinder. Find the Sun and the Moon symbol. Move the mouse cursor slowly from the first day of the month to the last. Notice the relationship between the Moon's phase and its elongation from the Sun in the sky.

diagram showing the elongation of the Moon in reference to it's phase

Perihelion

The point in the orbit of a body around the Sun when it is nearest to the Sun.

Most often used when talking about comets, but all objects in orbit around the Sun come to perihelion at some point in their orbit.

Perihelion distance is simply the distance between the Sun and an object when they are at their closest approach.

diagram showing an comet at perihelion

Aphelion

The point furthest from the Sun in the orbit of a body around the Sun. The opposite of perihelion.

Often used when talking about comets and asteroids, but also the planets and other objects in orbit around the Sun come to aphelion at some point in their orbit.

diagram showing an comet at aphelion