Archive for the ‘Solar System’ Category


Aberration of light is the apparent displacement of a star from it’s true position in the sky. It is caused by a combination of the motion of the Earth in orbit round the Sun (about 30 km per sec) and the finite velocity of light (299,792.5 km per sec or , if you prefer imperial units, 186,252.5 miles per second). The rotation of the Earth also gives rise to the aberration of starlight.

To understand aberration, we need to start off with a simple easy to understand example from the familiar world around us. Imagine you are in a parked car and you look out of the window and the falling rain. Imagine that there is no wind so the rain is falling vertically. As the driver pulls away and picks up speed to say 30mph, you notice that the rain is no longer falling vertically. Actually it is, but you are moving forwards, past the raindrops, thus greating the illusion that the rain is falling diagonally, slanting towards the back of the car.

OK, so back to the starlight. The Earth is moving forwards through space and, despite the high speed at which light travels, the starlight we see effectively is slanting backwards compared to the direction of movement of the Earth in its orbit. But the Earth moves in an ellipse round the sun so the direction of ‘slant’ of the light changes too. The net result is that if the precise position of a star is recorded throughout the year, it will be seen to describe a small ellipse around its ‘true’ position … the ‘true’ position being where the star would have been seen had the Earth been stationary.

There is also a very much smaller daily effect caused by the rotation of the Earth. This is called diurnal aberration.

The maximum displacement is 20.5 seconds of arc. This number is called the constant of aberration.  For a much more thorough treatment, including a discussion of relativity and aberation, click here.

Achondrite Meteorites

An achondrite is the name given to a type of stony meteorite containing very little iron or nickel and no chondrules. A chondrule is a rounded grain that originally formed as a molten droplet in Space.Achondrites have a basic mineralogiy i.e. similar to basalts. Because they are very similar to terrestrial basic igneous rocks, it is very difficult to locate them unless the landing area has been seen. According to reflection spectra studies, the majority of them are associated with the asteroid Vesta but there are a dozen or so subdivisions of various origins. One thing is certain though, because of the magmatic differentiation seen in these meteorites, they have come from a ‘differentiated’ object. Differentiation of magma (molten rock) occurs when a magma chamber is large enough and molten for long enough for crytals to have started to form. The heavier crystals sink slowly through the magma and a gradual chjange in composition can be seen after the magma has turned to rock. Because of their mode of formation, chondrules are not seen, hence their designation.

The public access area of the Hubble Space Telescope web site contains low resolution images of the surface of asteroid 4 Vesta if you are interested to see where most achondrites come from.

However, the achondrites are not exclusively from Vesta, the famous ‘Martian fossil’ meteorite found at Allan Hills in Antarctica is of this type. Other achondrites appear to be Lunar in origin. In terms of numbers, they are fairly rare and form about 4% of the known meteorites.

Achondrite Meteorite Falls

Station workers saw a fireball which fell on the Millbillillie estate in Western Australia. An object “with sparks coming off it” fell onto a plain to their north. No one went to have a look at the time but a decade later, in 1970, some remnants of an achondrite meteorite were found. Over the years, Aborigines walking the area have found other fragments, the largest fragment weighed in at 20kg and is on display in the Western Australia Museum. Others have been found in France, the USA, Mexico, Egypt, India and several other locations.


Aerolite – glue, baggage or meteorite?

Meteorite of course! But the word has been borrowed by glue manufacturers, insulation manufacturers, a steam train, trailer makers, golf companies, luggage makers and a whole host more!

The astronomical meaning of Aerolite is a stony meteorite. A stony meteorite is composed of silicate minerals and little metallic matter.

Aerolite meteorites can be categorised as chondrites or achondrites. Chondrites contain chondrules, achondrites don’t! A chondrule is a rounded grain.

Chondrites are thought to be very old objects indeed. The grains are believed to be the original matter of the Solar System. They were probably formed before the planets. They are chunks of asteroids and have inevitably been thermally altered but never melted, thus preserving their original granular appearance. It is possible that some of this class of meteorite contains material from before the solar system was formed.

An achondritic aerolite has been melted at some point in its history and contains no chondrules.

Jules Verne mentions the word aerolite in chapter 19 of his novel, From the Earth to the Moon; and, Round the Moon.


A feature of a planetary atmosphere. It gives rise to a faint glow which means that even on a planet with no artificial light, the night sky would not appear to be absolutely dark.


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.

Angular Distance

There are many ways to measure angular distance using your body …

The apparent distance between two celestial objects. It is measured in degrees, arcminutes (an arcminute is a 60th of a degree) and arcseconds (an arcsecond is a 60th of an arcminute). On average, the distance from your thumb tip to the tip of your little finger of your outstretched hand at arms length is 20 degrees. The width of your palm will be about 12 degrees and the width of the tip of your little finger is about 1 degree. The angular diameter of the Moon (and the Sun) is more or less 1/2 degree.

Angular Distance

Angular distance between two astronomical objects

An observer looks at two different objects. The angle between them can be measured e.g. by using a cross staff. This angle is the angular distance of the two objects in the sky. It is expressed in degrees, arcminutes and arcseconds.


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



One of the most noticeable mountain ranges on the Moon, next to Mare Imbrium (Sea of Showers).

The first of the Apollo missions to use the Lunar Rover, Apollo 15 landed close to the Appennines. The site was chosen as it was felt that it would be possible to sample rocks from deeper in the Moon’s crust than previos missions and also that the feature known as Hadley Rille, a structure resembling a lava tube such as those seen on Hawaii, could be explored.

The range itself is about 225 miles in length and rise to a height of about 14000 feet above the Mare Imbrium.


The apparent close approach of one celestial body to another. This is a line of sight effect.

Diagram showing an appulse

Diagram showing an appulse

Explanation of the diagram: The image on the right is what an observer sees. The diagram shows what is really going on. The orange circle represents a planet, the blue circle represents the Earth. As the planet travels round its orbit, to an observer on Earth, it will be seen to pass close to some stars. In reality, the two objects are separated by many light years. This is referred to as a ‘line of sight effect’.


The physical study of the planet Mars.

But why Areography? Mars has always been associated with the god of war. The Babylonians were keen astronomers and names Mars Negral. Negral is the Babylonian god of war. This appears to have carried over to other civilisations but it was the Roman name for the god of war that has ‘stuck’. Ares was the Greek god of war, the son of Zeuss and Hera. Ares is the root of this word … it does have a better ring to it than marsography!

No one can say for certain why each planet was names for a particular god, some say that it is because of the red colour of the planet in the night sky that reminded the ancients of blood and therefore battles. I guess it is also possible that one of the city states of the middle east was attacked when Mars appeared in the sky. The winners of the battle may have then decided that it was the god of war looking down on their efforts and helping? Who knows?

The first areographer of note was the Italian, Giovanni Schiaparelli, who started the myth of the canals of mars. The word he used to describe the features he had seen was ‘canali’ which properly translates as ‘channels’. That was in 1877 and people have been seeing all sorts of evidence of a former civilisation on Mars ever since!

As a result, there are many fiction and factual books concerning Mars …

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


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

Astronomical Twilight

The period after sunset when the sun has dropped between 12 and 18 degrees below the horizon.

It is the final phase of twilight, the other two phases being civil twilight (sunset to 6 degrees below the horizon) and nautical twilight (sun is between 6 and 12 degrees below the horizon).

Astronomical Unit

The mean (average) distance of the Earth to the Sun is termed 1 Astronomical Unit (1 AU). It is a convenient way of describing distances within our Solar System.

Nominally, 93 million miles or 150 million km. It is therefore a lot more convenient to use but even this mind-bogglingly enormous distance is inadequate to express the distance to galaxies so that is when parsecs are used.


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.

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