Astro Terms Simply Explained

Angle

Measured in degrees, minutes and seconds. There are 360 degrees in a full circle, roughly 180 from horizon to horizon. Each degree has 60 minutes, each minute 60 seconds.

A quick quiz. How big is the moon in angular degrees? 30 degrees, 10 degrees, 5 degrees, 1 degree or 30 minutes? (see further down for answer)

Asterism

Often confused with constellation. An asterism is a shape that the stars make on the sky. A good example is the Plough in the constellation Ursa Major.

A.U. or Astronomical Unit

One unit is the distance from the earth to the sun i.e. 93 million miles. It is easier to use AU to state a distance, as it is easier to write.

Barlow

A lens doubler or tripler. Put one of these lenses in between the telescope and eyepiece and the magnification is doubled or tripled. An outfit consisting of two eyepieces and a Barlow will give you four different magnifications with which to work.

Celestial Sphere

Does not actually exist. Man has imagined that the stars are on a glass sphere surrounding the Earth. This is because they appear to be at a fixed distance and never appear to move.

Circumpolar

This is a region of the sky that is always visible around the celestrial pole. This means that an object within this area does not set and can always be observed.

Constellation

An area of the sky. These are used to aid identification of an object's position without referring to positional coordinates. For example Saturn in November 1998 can be found in the constellation Pisces.

Double Star

A star with a companion. The two stars are circling each other round a point. Sometimes they both can be seen, sometimes a telescope will reveal the hidden partner. Try looking at the Plough. The middle star in the handle has a partner visible to the naked eye, use a telescope and a second partner can be seen. That's a double double!

Declination

The number of degrees above or below the celestrial equator in which an object is found. The celestrial equator is above the Earth's equator and is at zero degrees. The North celestrial pole above our North Pole is 90-degrees. See Telescope Parts.

The Ecliptic

A line through space through which the sun travels. The planets also follow this line closely. Look on a star chart to see where it goes. A bright object found on, or near, this line is probably one of the planets.

Eyepiece

The lens that you look through. They enlarge the image that the telescope produces. Changing the eyepiece will change the magnification. See Telescope Parts.

Focuser

Holds the eyepiece, racking it in and out to focus the image in view. See Telescope Parts.

Finderscope

A small telescope mounted on the side of the main telescope. This has a wide angle of view. Cross hairs in the middle, when aligned with the target, should mean that an enlarged view will be seen in the telescope's eyepiece. See Telescope Parts.

Focal Length

This is the distance from the lens, whether a lens or mirror, that an image of a target at infinity is brought to a focus. A short focal length is around 400mm a long focal length 1000mm and above.

Huygens, Kelner, Plossl

Types of eyepiece design in ascending order of complexity, performance and cost!

Light Year

Light travels at 300,000 km per second. A Light Year is the distance that light will travel in one year, 1.09 billion km.

Magnitude

An indication of how bright a star is. The dimmest seen by the naked eye has a value of 6. The brightest stars have values of Zero. Each magnitude is 2.5 times dimmer than the previous one, so a five magnitude difference means a star is 100 times dimmer.

Magnification

How much bigger is the view? A simple sum will tell you. On the actual telescope you will normally find written its focal length in millimetres. As an example, a telescope is inscribed with a figure of 900mm. If you divide this figure by the focal length of the a 9mm eyepiece (generally written on it) you will obtain the following result: 900mm divided by 9mm equals a magnification of 100 times.

Messier, Charles

Catalogued fuzzy objects in the sky when looking for comets in the eighteenth century. His catalogue is still in use today. Try looking at Messier's 31st object, M31, this is a large galaxy in Andromeda....a lovely target.

Meridian

A line directly above the observer drawn from the North Celestrial Pole due south to the equator. Quite often the time at which an object will pass through the meridian is given in astronomy magazines.

Nebulae

A cloud of glowing gas in space. The gas is either bright because a close star's light is reflecting off it or the powerful emissions from a star are making the nebula glow. Try looking at the Orion Nebula. It's amazing.

Newtonian

A reflector type of telescope invented by Sir Isaac Newton. The light enters an open tube travelling to the curved primary mirror at the base. The light bounces back up the tube coming to a focus. But before it reaches that point it is bounced through 90 degrees from a secondary flat mirror into the eyepiece near the top of the tube.

North Celestial Pole

A point in space about which all the stars seem to rotate, currently near the star Polaris. There is a similar point in the southern hemisphere.

Right Ascension

Measured in time. Hours, Minutes and Seconds, along the celestrial equator from west to east. Zero is a point in Pisces where the ecliptic and the celestrial equator cross. It is used in conjunction with the declination to locate an object in the heavens. See Telescope Parts.

The Terminator

The line of shadow on the moon. Markings on the moon stand out almost in 3-D near the shadow. Each night new sights are there to be seen.

Schmidt-Cassegrain

A variant of the Newtonian reflecting telescope but in this case the secondary mirror reflects the light back through a hole in the centre of the primary mirror into the eyepiece. This doubling of the reflection distance makes these telescopes shorter in length.

Refractor

A telescope design recognisable to everyone. A tube with a big glass lens at the front and the eyepiece at the opposite end.

Sky Map

A road atlas of the stars! Stars are represented with different sized dots for different brightness's, the bigger the brighter. The course of the planets is marked as is the Milky Way. A very useful addition for observing.

Star Diagonal

A mirror or prism that bends the light through 90 or 45 degrees making it easier to view objects directly overhead when using a refractor telescope. See Telescope Parts.

Star Hopping

An easy way of finding your way around the sky. Starting off at a known bright star the telescope is moved from star to star following patterns in the sky read from a star atlas until the final point is reached. Tours of the sky can be planned passing through as many interesting objects as possible.

Variable Star

A fascinating subject for amateur astronomers, and where they can still be of use to the professional astronomical community. Variables are stars that shine at different intensities at different times. Some go up and down by large amounts over a very short time, others take years.
Watch and note how the intensity of the star's shine changes by comparing it to close reference stars.

Answer to quiz. 30 minutes. It's always smaller than you imagine!