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Astronomy Glossary

Words appearing on these astronomy pages in bold are (normally) listed in the glossary.

Black-body Radiation

When things get hot, they glow. The colour they glow is directly related to the temperature. This is why a stove element glows red and a candle flame is orange-yellow.


It's rarely that a really impressive comet is visible, so when they do appear it's worth taking the time to have a look.


When the Sun and Moon are in the same direction and line up perfectly with Earth, the Moon obsures the Sun – a solar eclipse.

When the Moon is on the opposite side of Earth to the Sun, the Moon enters Earth's shadow – a lunar eclipse.



Iridium Flares

Surprisingly common, all you have to do to see these is be looking in the right direction at the right time. Some are even visble during the day.


Magnitude is the brightness of a star, either apparent or absolute. Some examples are provided.

(Magnitude is also used to describe the totality of an eclipse.)


Also known as shooting stars, it was only a little over 200 years ago that they were first seriously thought of as being falling rocks. Prior to that they were thought of as being atmospheric phenomena.

About 500 meteors land on Earth's surface a year, whereupon they are called meteorites.

Minor Planets

There are lots of small bodies orbiting the Sun that are not big enough or commanding enough to be considered planets. Dwarf planets, asteroids, trojaens, centaurs, plutinos, and many more.


The biggest bodies orbiting the Sun. There are three qualifications for being a planet.

Indirect evidence is growing for Planet Nine. If found, it will meet the three present criteria. (It's thought to be inclined at 30° to the ecliptic; not a problem for it being a planet.)


Planetary Conjunctions

It's not hard to spot the brighter planets by themselves, but when they appear together in the same part of the sky it's something special.


When one celestial body moves across the face of another.

Solar System

There much more in our stellar neighbourhood than planets and a few asteroids.

Spectral Types

The colour of a star is directly related to how hot it is, and is the main way that stars are classified.


A star is a big hot ball, mostly made of hydrogen, and converting that hydrogen to helium by nuclear fusion.


The really big hot ball at (or near!) the centre of the Solar System.

Weird Stars

Not every star out there is as normal as the Sun. For a long time Sirius has been a reasonably well behaved bright blue-white star, but historical records show that for a while it had a habit of changing to a distinct red colour every few decades.

And if that's not weird enough, black holes get a mention too.