Posts Tagged ‘planet’

Cassini Division

The principal division in Saturn’s ring system, separating ring A from ring B.


The gaseous mantle surrounding a planet, star or other astronomical body.

It is thought that the atmosphere of the Earth is a secondary atmosphere. The theory is that the original (primary) atmosphere was lost during the T-Tauri stage of the Sun’s evolution. Volcanoes gradually replaced this with an atmosphere of methane, carbon dioxide and water vapour. The current atmosphere evolved from this. The oceans were formed as the water vapour condensed as Earth cooled down. When plants containing chlorophyll evolved, they used the carbon dioxide for photosynthesis and introduced oxygen into our atmosphere. Ultraviolet radiation converted some oxygen to ozone in the upper atmosphere and was therefore absorbed and with the development of the ozone layer, life on earth could colonise the land.

Ashen Light

When the plant Venus appears as a crescent, the night side sometimes appears dimly luminous. This is the Ashen light, also known as the Ashen Glow.

It is one of the many unexplained mysteries of the Solar system. It was first noted by an Italian astronomer Giovanni Battista Riccioli way back in 1643 and has been seen by many astronomers since – including Big Bill (Herschel). professionals operating the Keck telescope and of course the late great Patrick Moore. No photographic images exist of the Ashen Light and many professional astronomers have never seen this phenomenon, however, it is accepted as being a genuine thing.

Various theories have been proposed to explain the Ashen Light including atmospheric ionisation, lightning and Venusian inhabitants either carrying out slash and burn agriculture or celebrating the crowning of a new ruler!

This article by Jenny Winder sums it all up nicely


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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