Archive for the ‘Solar System’ Category

Atmospheric Refraction

This is the bending of light the Earth’s atmosphere. This causes an increase in the apparent height (altitude) of an object above the horizon. This increase is zero at the zenith (because the observer is looking out from the Earth at 90 degrees) to about half a degree (the width of the full Moon) at the horizon.


The smallest unit of a chemical element which retains its own characteristics. An excellent book on the atoms and Einstein’s theory of relativity is “Mr Tompkins in paperback” by George Gamow.

When atoms combine in nuclear reactions, such as the nuclear fusion at the heart of stars, a small amount of mass is ‘lost’. It is not in fact truly lost, the missing matter is converted into energy as described by Einstein’s fameous equation.

Bailey’s Beads

Brilliant points of light along the edge of the moon disc, just at the start or the end of totality of a total solar eclipse.When caught on camera they are spectacular and can give the diamond ring effect.

They are caused by the irregularities at the edge of the Moon’s disc when seen from Earth. These irregularities are in fact the hills and valleys of the Moon. It is the valleys that allow the light from the Sun to reach the observer when the Moon obscures the Sun.


How to observe eclipses

Black Drop

An appearance seen at the end of second contact and at the start of third contact of a transit of Venus. As the planet moves across the Sun’s disc it seems to draw a strip of blackness after it.  It makes measurement of the exact time of these contacts difficult to measure accurately.

The black drop has been known since telescopic observation of the transits of Venus and Mercury began with several explanations put forward. It now appears, according to the AAS, that is is a combination of the Sun’s limb darkening and the telescope that produces this effect.



An extremely bright meteor, also called a fireball.

Cassini Division

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

Celestial Equator

A great circle on the celestial sphere that is midway between the two poles of rotation. It really amounts to the projection of the Earth’s equator onto the sky. It can be thought of as an imaginary ring, 90 degrees from either pole in the sky.

Celestial Longitude

The angle that is measured eastward along the ecliptic, from the vernal equinox, to the foot of a circle that is perpendicular to the ecliptic and passing through the object.

Celestial Sphere

This does not actually exist. Early humans believed that stars were fixed to a crystal sphere in the sky, at a great distance from Earth. This is because there is no sense of distance in the night sky and the stars always seemed to be fixed in position relative to each other. The idea is a handy one when dealing with positioning and angles between objects in the sky. See also celestial latitude, longitude, azimuth.


The part of the sun’s atmosphere lying above the bright photosphere, but below the corona. The temperature rises to around 10,000K in this part of the Sun’s atmosphere. It can be seen during times of total eclipse as a red coloured rim around the Sun. The red colour is caused by the emission of red light from the hydrogen which makes up the majority of the Sun. Hydrogen emits at many other frequencies, however, only the Balmer series of emissions are in the visible and there is a strong red line hence the red colour.

For a few second either side of totality, a ‘flash spectrum’ can be obtained from the chromosphere. The emission lines largely correspond to the dark absorption lines that are recorded on spectra of the photosphere.


A cluster is a group of stars whose members are genuinely associated. A cluster of stars is formed from the same gas/dust cloud. There are two main types: open and globular.

Cluster is also the name given to a group of four spacecraft that have been placed in Earth orbit in order to study the interactions between particles ejected from the Sun and the Earth’s magnetic field. The first attempt to put Cluster into space in 1996 failed when the Ariane rocket exploded. Oops.


There are several astronomical meanings. It can be used to describe the hazy looking patch that surrounds the nucleus of a comet or the blurred effect surrounding the images of stars on a photographic plate, or in the observers field of view in a telescope (or binoculars) due to defects in the lenses.


A body of our Solar System. Comets are composed of rocks, dust and ices. This composition was first recognised by the astronomer Fred Whipple (Lawrence Frederick Whipple: November 5, 1906 – August 30, 2004) who described them as an ‘icy conglomerate’, translated for the press as being like a ‘dirty snowball’. As they approach the Sun, the ices begin to evaporate forming a coma and one or more tails. The particles frozen in with the ices are carried off as the ices blast their way off the surface and it is the Sun’s light reflecting off these that enable the tail to be seen. The dust tail is often curved whilst the second tail, if present, is straight  and usually much fainter; it it made up from plasma – charged particles of gas.

Long period comets are believed to come from the Oort cloud and possibly contain material from the earliest days of the Solar System.

Short period comets are believed to originate from the Kuiper Belt.

The first spacecraft to visit a comet was the European Space Agency’s firt deep space mission – Giotto. This was launched to take a closer look at Halley’s comet. It collected samples from the tail and managed to take some images of the nucleus, showing clearly that the gas and dust erupted from the surface of comets as jets. Despite being damaged, the craft survived to visit a second comet – Grigg-Skjellerup.



This is when a planet has the same longitude as the Sun. A planet can only be observed at conjunction if there is a total eclipse or if it transits the Sun. Unlike the other planets, Mercury and Venus have two types of conjunction, inferior when the planet lies between the Sun and the Earth, superior when it lies on the opposite side of the Sun to the Earth. At inferior conjunction, if the inclination of the orbit of Mercury or Venus coincide with that of the Earth, a transit is observed. These are rare phenomena.

Copernican System

Although this sounds like some far-flung solar system in a science fiction series, it isn’t! It is a description of how the Solar System is arranged.

The Copernican system is in fact the system proposed (in 1543) by Nicholas Copernicus in which the Sun is the central body, with the Earth and the other bodies moving around it. This model superseded the Ptolemaic system which had persisted for nearly 2000 years. Copernicus’ idea was not new, it had been proposed in about 300 BC by Aristarchus of Samos, a Greek philosopher. Both astronomers have Lunar craters named after them.

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