Posts Tagged ‘light’

Dark Adapted

This is what every astronomer’s eyes should become before they begin observing. On leaving a brightly lit area and entering a dark area, you will notice a rapid increase in visibility over a short period of time. However, it takes about 20 – 30 minutes to become fully dark adapted. In low light levels, the chemicals in the eye increase the ability to see faint objects. See also the Purkinje effect

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.

Colour Index

A measure of a star’s colour, which helps astronomers to tell its surface temperature. It is the difference between the magnitude of a star measured in two different areas of the spectrum. The areas are B (blue), V (violet) and U (ultraviolet) regions. The B-V is the most common index used and is close to zero for a white star. It is extremely useful in the classification of stars, it can tell astronomers if the star is a main sequence star, a giant star or a supergiant star.

The intrinsic colour index is a modified form that has been corrected for interstellar extinction.


Binary Star

double star in which the components orbit one another. Some binary systems orbit so close to one-another the two stars are distorted by each other’s gravity.

If the two stars eclipse one-another when seen from the Earth, the light changes regularly and predictably over a period of time. This type of binary is an eclipsing binary and is a type of variable star.

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.


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.

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