Reference
Reference
This glossary explains, without complex formulas, the words that keep coming up whenever people talk about the night sky. Terms are grouped by theme: the sky itself, the Moon and planets, and observing practice. Come back here whenever you meet an unfamiliar term in an article or on a star map.
Once a phenomenon has a name, it is easier to notice. Learn the word "terminator" and you will start looking for the strip on the Moon where the terrain stands out best.
The sky
- Constellation
- An area of the sky with defined boundaries, together with all the stars inside it. The International Astronomical Union recognises 88 constellations, which together cover the whole sky.
- Asterism
- A noticeable pattern of stars that is not an official constellation in its own right. The best-known example is the Plough, or Big Dipper, which is part of the constellation Ursa Major.
- Magnitude
- A measure of how bright an object appears. The lower the value, the brighter the object; for the brightest objects it can even be negative. Faint stars are the first to disappear under light pollution.
- Ecliptic
- The apparent path of the Sun among the stars over the year. The Moon and planets always stay close to this line, so that is where to look for them.
The Moon and planets
- Phase
- The part of the Moon's disc lit by the Sun as seen from Earth. The Moon goes from new Moon through first quarter to full Moon and last quarter, and then the cycle repeats.
- Terminator
- The boundary between the lit and dark parts of the Moon or a planet. Shadows near it are long, so craters and mountains stand out in especially strong relief.
- Synodic month
- A full cycle of lunar phases, for example from one new Moon to the next. On average it lasts about 29.5 days.
- Planet
- A large body orbiting the Sun. Several planets are visible without equipment as bright points that slowly move against the background stars.
Observing
- Dark adaptation
- The gradual adjustment of the eyes to darkness, after which faint stars become visible. A glance at bright light quickly undoes it, which is why a red torch comes in handy.
- Light pollution
- Skyglow caused by street lighting, illuminated signs and windows. Because of it you see far fewer stars in the city than away from towns and villages.
- Meteor shower
- A period when meteors appear more often than usual and seem to fly out of one area of the sky. An example is the summer Perseids, which peak around 12–13 August.
- Radiant
- The point in the sky from which the meteors of one shower seem to radiate. A shower is usually named after the constellation where its radiant lies, but it is best to watch a wide area of sky around it.
Where these words come in handy
- Guides
How to photograph the Moon with a smartphone: preparation, steps, mistakes and a checklist. Plus the article catalogue.
- Questions
Short answers about naked-eye observing, binoculars and telescopes, smartphones and photographing the sky.
- Sections
Four topics: how to observe, constellations and stars, the Moon with planets and meteors, smartphones and photography.
- Collections
Reading paths: your first month of stargazing, the sky with children, observing from the city.
Read next
- Questions
Short answers about naked-eye observing, binoculars and telescopes, smartphones and photographing the sky.
- Guides
How to photograph the Moon with a smartphone: preparation, steps, mistakes and a checklist. Plus the article catalogue.
- Sections
Four topics: how to observe, constellations and stars, the Moon with planets and meteors, smartphones and photography.
Sources
- International Astronomical Union: list of constellations
- Encyclopaedic sources on astronomy
Information is current as of the date this page was prepared. What you can actually see in the sky tonight depends on the date, your location and the weather, so check an up-to-date astronomical calendar before heading out.