Posts Tagged ‘solar system’

Escape Velocity

The escape velocity is the velocity required by an object (spacecraft, gas molecule, photon of light or whatever) to escape the gravitational pull of an astronomical body.

The more massive an astronomical body is, the greater the escape velocity will be. This is because a gravitational field is a property of mass. Another factor that needs to be taken into account is how concentrated the mass is – for two bodies with the same mass but different diameters, the one with the smaller diameter would have the greater escape velocity. This is a simplification of escape velocity, for a more advanced treatment, including the maths, see the Wikipedia article.

When the Americans landed men on the moon, the Lunar Module engines needed to develop considerably less thrust to return the astronauts to the Earth than the original Saturn V rocket since the Moon has a much smaller gravitational field. Had this not been the case, the whole process would have been considerably more difficult, if not impossible.

Another factor that worked in the Lunar astronaut’s favour is the lack of an atmosphere on the Moon. In direct sunlight, the surface of the Moon can reach temperatures in excess of 100 degrees Celsius. Any gas molecules near the surface will gain kinetic energy i.e. move faster. The actual speed of a gas molecule also depends on other factors such as its atomic or molecular mass but take for example, oxygen. At a temperature of 100 Celsius, the average velocity will be in excess of 5 km per second so the Lunar gravity will be unable to hang on to it. Without an atmosphere to create drag, the thrust from the engine will be unopposed so much less fuel would be needed to propel the Lunar module up into orbit. From there, with just a little extra effort, escape velocity could be achieved for the Lunar Module/Command Module combination.

For the Earth, escape velocity is 11.2 km per second whilst for the Moon, it is just 2.4 km per second. The largest planet of the Solar System, Jupiter, has an escape velocity of almost 60 km per second whilst the Sun’s escape velocity is over 600km per second.

The gravitational attraction of a black hole is so great that an object would need to be travelling faster than the speed of light in order to escape. That is why black holes are essentially invisible, even photons of light cannot travel fast enough to escape the immense pull of the gravity of a black hole.


Elongation is a term applied to Mercury and Venus.

When seen from Earth, the two inferior planets appear to become closer or further away from the Sun as they move round their orbits. When either planet reaches its greatest angular distance from the Sun, they are said to have reached eastern (or western) elongation.

Elongation is not a fixed point in their orbits as it is an observational factor that depends on where the Earth is in its orbit as well as the position of Mercury and Venus in theirs.

Counter Glow

The English name for Gegenschein. It is a faint oval patch of light that is very difficult to see, you need a clear and moonless night. It is at the antisolar point i.e. exactly opposite the Sun.

It is apparently best seen when the ecliptic is at its highest above the horizon (midwinter from the northern hemisphere and vice versa for the southern hemisphere). It is not well understood but is thought to be caused by the scattering of sunlight from dust in the main plane of the Solar System. Sometimes it can be seen to be joined to the zodiacal light by a parallel sided beam of light. I think that this beam is called the zodiacal band but cannot be 100% sure. I also believe that it is larger in the tropics than in the temperate zones. Click here to visit the NASA website and view a picture of the gegenschein.


The corona is a huge envelope of gases that form the outer portion of the Sun. It is seen as faint extensions of the Sun’s outer atmosphere during total eclipses and can be observed using a corongraph, a device that is used to create an artificial total solar eclipse.

The shape of the corona varies with the sunspot cycle. At maximum, it is much more even. At minimum, it is seen to be much more irregular with polar tufts, equatorial streamers and plumes being visible.

It has a temperature of between 1 and 2 million K and naturally at that temperature, is a plasma, for example, iron has been identified but with half of its electrons stripped away. It is indeed a very high energy environment. The corona of the Sun is the origin of the solar wind. An estimated 3 million tonnes of material is ejected from the Sun each second.

This material is experienced as the solar wind with ‘gusts’ of up to several hundred km per sec. The Earth is protected from this bombardment of charged particles by its magnetic field which shunts them into the Van Allen Belts. When these overflow at the poles, they create the aurora borealis and aurora australis.

The solar wind appears to be strongest where it flows from coronal holes. These are areas of lower temperature punched through the corona by the powerful magnetic fields of the Sun.

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.


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.


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.



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.

Sun Grazer

A description applied to a comet that will inevitably disappoint the astronomical world – it is a synonym of ‘the comet of the century’! A Sun Grazer comet is one which passes close to the Sun and therefore offers great potential to become extremely bright with a massive tail. The problem is that as a sun grazer approaches perihelion, the temperatures and gravitational pull will destroy it. Every once in a while, a Sun grazer actually survives and dos indeed become a ‘comet of the century’.

Sun grazers were recognised in the 18th century by the astronomer Heinrich Kreutz working in the last two decades of the 19th century. He noticed that the sun grazers had very similar orbits and postulated they came from a large one that broke up. Very few sun grazers have been discovered from the surface of the Earth. Since the end of the 1970s, satellites using coronographs have been used to discover large numbers of this type of object. In fact, well over 700 have been discovered in thae last 30 years, about 500 being attributed to the ‘Kreutz Group’ of comets, following the orbit he determined all those years ago. The remaining comets seem to belong to 3 separate groups but those of the Kreutz group are approach the Sun much closer than the others. That is probably the reason he saw them as they become brighter before meeting their evaporative end.

An astrophysics chappie, probably an American I guess, has said that a big sun grazer could create a massive solar flare that knocks out electronics on earth.


Cassini Division

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

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.


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


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.

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.

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).

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