Posts Tagged ‘asteroid’


Asteroids are also known as minor planets.

These are relatively small bodies that orbit the Sun. In the early days of Solar System astronomy, it was believed that they existed in the space between the orbits of Mars and Jupiter. As knowledge progressed, asteroids (or minor planets or planetoids as they are also known) have been found throughout the Solar System. The Trojan Asteroid follow the orbit of Jupiter, one group ahead and one group following.

They are though to be material of the Solar System that never accreted to form a planet. There are others who believe that they are the remains of a somehow destroyed planet that lay between Mars and Jupiter.

In recent years it has been realised that there are a number of asteroids that cross the orbit of the Earth. Some of these are quite large and would cause great problems if they impacted the Earth. Life on this planet could conceivable be eradicated by a large enough impact. It is now generally accepted that mass extinctions of life in the past e.g. the demise of the dinosaurs, could have been caused by such an impact. There is now an observing programme to locate and evaluate the dangers of this hazard (NEAR). I have put some links to books on the subject below.

A more comprehensive treatment of asteroids can be found at


When something is orbiting the Sun, this is the point of the orbit that is furthest from the Sun. It applies to anything in solar orbit – planet, comet, minor planet, dust particle …

For the Earth, aphelion is around July 4th, when the Northern Hemisphere is in summer.  The word aphelion derives from the Greek words, apo meaning away, off, apart and Helios (the Greek god of the sun).

The reason why the this occurs is because orbits are elliptical and not circular. Kepler realised this and published the information in 1609. An ellipse has two foci which I suppose can be regarded as the equivalent of the centre of a circle and are used to construct the ellipse. In terms of an orbit, the Sun sits at one of the foci therefore as a body orbits the Sun it will have a varying distance.

Illustration of aphelion and perihelion

Illustration of aphelion and perihelion

There is a further explanation and animation at



Albedo of an astronomical body

A measure of how reflective a body is. Albedo is expressed as a percentage, the higher the percentage, the higher the albedo and therefore the more reflective the object is.

On the face of it, albedo is a straightforward thing, however, the albedo of a planet varies from place to place. Dark surfaces absorb more light than light surfaces. Rough surfaces scatter light in all directions and therfore reflect less back to the observer. Thus when talking about the albedo of an astronomical body, one generally means the average albedo.

Cloudy planets like the gas giants and Venus have high albedos because clods are good reflectors whilst the rocky planets have lower albedos.

The light reaching your eye from an object is reduced thr further it has to travel so the albedo at the edge of a planet is less than at the centre, assuming the same surface composition. Accurate measurements of albedo need to take this into account and so there are two types of albedo, spherical and geometrical. The latter assumes the planet, asteroid, moon or whatever is a uniform sphere whilst the latter compares the reflecting power of the object with that of a flat white disc of the same diameter and distance as the object.

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