Astronomy Basics Basic Astronomy Terms Basic Guide to Astronomy Astronomy Terms Basic Astronomy

Our fascination with the night sky and what is ‘out there’ is as old as the human race itself. A basic knowledge of what you’re looking at when you simply step outside and look up at a dark, star filled sky can be a rewarding and satisfying knowledge to have, bringing with it a sense of where we are in the big scheme of things.

This guide to basic astronomical terminology is aimed at the newcomer to star-gazing, with easy to read definitions of common astronomical objects, movements, distances used in astronomy, and a very basic guide to finding objects in the night sky using celestial co-ordinates.

What we can see in the night sky.

What we see in our night sky is usually the visible light directly produced by objects such as stars which generate their own light, but we are also able to see planets and other objects because we can see light from our own sun being reflected back to us from those bodies.

The brightness of an object is given a MAGNITUDE. The lower the magnitude number, the brighter the object. For instance, a star with a magnitude of 1 is brighter than one of magnitude 2.

Now on to our solar system, some of the celestial bodies visible to the naked eye in the night sky, and then further out into our universe.

SOLAR SYSTEM: A solar system is a star and the planets or other bodies that orbit it (move around it). Our Sun (named Sol) is the centre of our solar system. Earth, the planets we know and the comets and asteroids we see all orbit our Sun.

STAR: A star is a dense ball of spinning gases, primarily hydrogen and helium that emits light as part of energy released by the nuclear fusion that is going on within it. Our Sun is a star.

PLANET: A planet is large celestial body that has enough mass to have a near round shape, caused by it absorbing most of the dust and debris that was originally around it. A planet orbits a star, and differs from a moon in that it does not orbit another planet.

MOON/SATELLITE: A moon is a celestial body that orbits a planet as that planet orbits a star. A moon is also sometimes referred to as a satellite, although a satellite today can also be a man-made object set in orbit around our earth or even around other bodies in our solar system.

COMET: A comet is a body that orbits the Sun, and is only generally visible during the closest part of its orbit, its perihelion. Comets are thought to have a solid nucleus. When close to the Sun though and being heated by it, comets emit a coma (gas surround) and a vapor ‘tail’ of gas and debris. Contrary to a surprisingly common misconception, comets do not move across the sky as you watch. They appear fixed in our skies, as do stars and planets, only traversing the sky as the Earth itself revolves on its axis, which is only discernible to us over a long period of time.

ASTEROID (also called minor planets or planetoids): Asteroids are generally not visible to the naked eye but are incredibly numerous in our solar system, particularly in the ‘asteroid belt’ located between the orbits of Mars and Jupiter. They can be any solid, generally irregular shaped body that orbits the Sun. They can vary in size anywhere from a few metres to a few hundred kilometres in diameter.

METEOR (also known as a falling or shooting star): A meteoroid is a small piece of dust or a body moving in space that is smaller than an asteroid. When these enter Earth’s atmosphere at high speed and are heated or burn up, they produce a streak of light across the sky, this is known as a meteor. Meteor ‘showers’ can also be seen on occasion, when the Earth passes through the debris trails left behind in space by comets.

GALAXY: Moving outward in distance from our solar system, we encounter galaxies and other stars in the universe. Our solar system is part of the Milky Way galaxy, a group of many millions of stars and solar systems that themselves orbit one central galactic point. There are countless galaxies in the universe we have so far been able to see, and doubtless many more beyond.

CONSTELLATIONS: Our ancestors, seeing patterns among the stars and galaxies, gave names to those patterns relating to mythological or revered animals, or gods or significant images in their cultures. Well known constellations include Ursa Major (also known as the Great Bear or The Plough or Big Dipper), and Orion, the hunter, with his most noticed 3 bright points creating his belt and another three bright points creating the dagger hanging from his belt.

Distances in space.

The universe is so incredibly vast that units of measurement we use on Earth become lost and almost incomprehensible when we move out into space. In our own solar system, we use a unit known as an ASTRONOMICAL UNIT or AU to measure distance. 1 AU is the mean distance between the Earth and the Sun, approximately 150 million kilometres. Therefore, we are on average 1 AU from the Sun. The Oort cloud, where it is believed comets originate, is believed to be some 50,000 or maybe even 100,000 AU from us.

When we start to move out into deep space, the distances are even greater, and become even more incomprehensible, so further units of measurement were devised by which we could more easily understand and calculate the distances involved. The LIGHT YEAR (abbreviated to ly) is used to measure distances outside of our solar system. A light year is the distance that light will travel in one year. It is equivalent to approximately 10 trillion kilometres.

For even greater distances, the PARSEC (abbreviated pc) is used, this is equivalent to approximately 3.26 light years. The parsec is the preferred unit of measurement to astronomers for reasons a little too complicated to venture into for a basic guide such as this, but it relates to the ease of measurement when related to actual movement of the Earth in space.

Movement of objects in space.

ORBIT: An objects orbit is the path it takes as it swings around the object it circles. One orbit is one complete circling of that object. Earth takes approximately 365 days to orbit the Sun. Our Moon takes approximately 28 days to orbit the Earth.

PERIHELION: This relates to distance from the Sun. An objects perihelion is the point at which it is closest to the Sun.

APHELION: Again relating to distance from the Sun, an objects aphelion is the point at which it is farthest from the Sun.

PERIGEE: This relates to distances of objects from Earth. An objects perigee is the point at which it is closest to the Earth.

APOGEE: Again relating to distance from the Earth, an objects apogee is the point at which it is furthest from the Earth.

Mapping the sky and locating stars and other objects.

If you lay on your back and look at the night sky, it can be imagined as a giant dome that covers us from horizon to horizon. If you then imagine the Earth were no longer there, and you were hanging in mid air, then you would have the same dome of sky underneath you, completing a full sphere around you. This is known as the CELESTIAL SPHERE, and all stars and objects in the night sky are given positions or co-ordinates on this sphere.

Just as the Earth has lines of latitude and longitude which allow us to provide a co-ordinate for any point on it’s surface, so, with the celestial sphere, latitude is known as DECLINATION and longitude is known as RIGHT ASCENSION. These are the basis for all celestial co-ordinates given to stars and objects as seen from Earth.

The CELESTIAL EQUATOR may be likened to the Earth’s equator in that it splits the northern hemisphere from the southern hemisphere of the celestial sphere. When looking at celestial co-ordinates, a plus sign (+) indicates north of the celestial equator, and a minus sign (–) indicates south of the celestial equator.

In general star-gazing terms though, an objects position in the sky can often be described by its proximity to a constellation, so getting to know your constellations can be very beneficial.