Posts Tagged ‘observation’


A distinctive group of stars that is not one of the recognised constellations.

The Plough is a cracking good example, it is not a constellation, it is an asterism that is part of the constellation Ursa Major. Another well known asterism, brought to popularity by Patrick Moore during the early days of his long-running TV astronomy programme ‘The Sky at Night’ is the Summer triangle of Altair, Deneb and Vega. I think that southern hemisphere astronomers refer to it as the ‘Northern Triangle’.

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


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 …


An alidade is a sighting device that is used to measure angles.

The earliest alidades were simply long sticks with sighting slots. They enabled early astronomers to measure the relative angles between stars and thus to produce the earliest star maps.

Over the years they became more and more sophisticated and incorporated small telescopes, for example, as used in the modern theodolite by terrestrial surveyors.

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.

Airy Disc

Airy Disc – limiting what you can see

Larger telescopes can see smaller objects. They can also resolve closer binary stars and finer details. Why? Magnification? No, it is all to do with the Airy disc …

The apparent size of a star’s disc produced by diffraction effects in an optical telescope. No matter how well made a telescope is, it will never be perfect. The light passing through a telescope will be diffracted. The diffraction creates a sequence of rings of decreasing brightness. The central ring is the brightest and it is called the Airy disc.

The Airy disc has an inverse relationship to the aperture of the telescope. The larger the telescope, the smaller the Airy disc. The size of the Airy disc limits the resolution of a telescope.

In a refracting telescope, about 84% of the starlight reaching the telescope goes into the airy disc, the other 16% forms diffraction rings around the disc, degrading the image and limiting the resolution. Less light makes it into the Airy disc in a reflecting telescope because of the presence of the secondary mirror. It is for that reason that a reflecting telescope of a given aperture will always out perform a reflector of the same quality.

George Airy was the 7th Astronomer Royal of the UK and it is named for him. Why? Because he was the chap who worked all this out of course! But he wasn’t the first to observe the phenomenon, that honour falls to John Herschel. Or at least it was Herschel who first described it.

But how does the Airy disc arise. See the Wikipedia article for a thorough mathematical explanation.

For a simple non-mathematical treatment this will have to do. Most readers of this article will appreciate that diffraction occurs when light passes through holes of a comparable size to its wavelength. It’s not just holes that create diffraction effects. Edges can do that too. So because a lens or a mirror has a finite size, it also has an edge.  It is because of the presence of the edge that diffraction takes place.

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