All About Space

Jargon buster



A conjunctio­n is an alignment of objects at the same celestial longitude. The conjunctio­n of the Moon and the planets is determined with reference to the Sun. A planet is in conjunctio­n with the Sun when it and Earth are aligned on opposite sides of the Sun.

Right Ascension (RA)

Right Ascension is to the sky what longitude is to the surface of the Earth, correspond­ing to east and west directions. It is measured in hours, minutes and seconds since, as the Earth rotates on its axis, we see different parts of the sky throughout the night.

Declinatio­n (Dec)

This tells you how high an object will rise in the sky. Like Earth’s latitude, Dec measures north and south. It’s measured in degrees, arc-minutes and arcseconds. There are 60 arcseconds in an arc-minute and there are 60 arcminutes in a degree.


An object’s magnitude tells you how bright it appears from Earth. In astronomy, magnitudes are represente­d on a numbered scale. The lower the number, the brighter the object. So, a magnitude of -1 is brighter than an object with a magnitude of +2.


When a celestial body is in line with the Earth and Sun. During opposition, an object is visible for the whole night, rising at sunset and setting at sunrise. At this point in its orbit, the celestial object is closest to Earth, making it appear bigger and brighter.

Greatest elongation

When the inner planets, Mercury and Venus, are at their maximum distance from the Sun. During greatest elongation, the inner planets can be observed as evening stars at greatest eastern elongation­s and as morning stars during western elongation­s.

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