Archive for the ‘Observing’ Category

Exit Pupil

The exit pupil is the image formed by the eyepiece of an optical instrument.

At low magnifications, the exit pupil is wider than at higher magnifications.


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.


This is the plane of the Earth’s orbit projected onto the celestial sphere. Effectively this is the apparent path of the Sun through the sky. Since the main planets of the solar system orbit in more or less the same plane, give or take a few degrees, they are always found close to the ecliptic.


The obscuring of one celestial body by another either by passing directly in front of it or by casting a shadow. Solar eclipses can be total (whole Sun is obscured), partial (part of the Sun is obscured) or annular (a complete ring of sunlight is seen). Lunar eclipses are either total or partial. Quite often, during a total lunar eclipse, the moon is seen to become a coppery colour due to light refracting through the Earth’s atmosphere.

Book: Africa and Madagascar Total Eclipse 2001 & 2002


A lens that comprises two elements glued together in order to reduce chromatic aberration. One of the elements is made from crown glass and the other from flint glass. They work well if the light is travelling parallel to the lens’s axis but performance is worse if it is not, in which case multi-element lenses should be employed.

Double Star

Double star – when you observe a star through a telescope or binoculars, it appears to be two stars as opposed to one when seen with the naked eye.

Sometimes this is a line of sight effect where the two stars are in reality totally unconnected with each other. In other cases the two stars are a genuine pair, orbiting one another, in which case they are referred to as a binary star.

Stars that are only connected by line of site are referred to as optical doubles or visual doubles.

Double Star Programme is also the designation of the fist ever collaboration between the Chinese and the European space agencies. It ran from early 2004 to late 2007. Two Chinese satellites were launched into orbits at 90 degrees to one another in order to study ‘global physical processes in Earth’s magnetic environment and its response to solar disturbances’ (esa). Working in conjunction with the existing Cluster satellites, Double Star Programme has msde several new discoveries, notably ‘Space is fizzy’ (density holes in the Solar wind in the region of the bow shock), Chorus emissions found at a greater distance (areas where high energy particles that can damage electronic equipment) and something called neutral sheet oscillations in the magnetotail.

Doppler Effect

The classic example is the change in tone of the noise of a vehicle engine as it approaches, passes and leaves the observer. As the vehicle approaches, more sound waves per second enter the ears of the observer. The sound appears to be higher pitched that if the vehicle and observer were stationary. This is because more sound waves per second = a higher frequency and higher frequency sound is heard as a higher pitch.

As the vehicle passes the observer and moves away, the engine tone is heard to drop. This is because slightly fewer waves per second enter the ear. Thus, the tone is heard to fall (lower frequency).

The Doppler effect was once used by the emergency services to help them to get through traffic. The first ‘sirens’ were bells, then we had the ‘me-ma’, a two fixed tone siren. Both of these were subject to the Doppler effect and drivers could recognise from which direction an emergency vehicle was approaching. Then some bright spark (read: complete and utter plonker) decided that American sirens were somehow better. These have a continuously changing tone so it is now impossible to tell the direction the emergency vehicle is approaching until it is really close. A brilliant way to make it more difficult to get through traffic and a perfect example of the detrimental way that adopting aspects of the US culture affects us here in the UK. It is as bad as those people who reply ‘I’m good’ when asked the question ‘How are you’ which grammatically makes no sense whatsoever.

Anyway, back to the Doppler effect in connection with astronomy. The same happens to all waves, including light waves and radio waves. In the case of light waves, higher frequencies are bluer and lower frequencies are redder. An object with a negative radial velocity (moving towards the Solar System) will be blue shifted and vice-versa will be red shifted. The amount of red-shift is used to determine the distance of distant galaxies. The Doppler effect causes emission and absorption lines of a spectrum to be shifted from their normal position. The faster an object approaches or receded, the further the lines will be displaced.



The angle north or south of the celestial equator to a star. It is one  of the two co-ordinates used to describe the position of an astronomical object on the so-called celestial sphere. Together with Right Ascension, declination describes where in the sky an object can be found.

GoTo telescopes are programmed with thousands of celestial co-ordinates and make it extremely easy to find an object. But where is the satisfaction in that? For an amateur astronomer, finding a faint object for yourself is extremely satisfying, users of telescopes with the RA and declination pre-programmed are missing out on half of the fun and can only really be called stargazers, not astronomy enthusiasts. Sadly, the marketing machine of telescope manufacturers suggests otherwise – they just want to get as much of your cash as they can. Cynical, aren’t I?

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


The maximum altitude of a celestial body above the horizon. In other words, when an astronomical body transits the meridian.


A pattern of stars named after an object, animal or mythical person. The stars in a constellation have no physical connection, they are a line of sight effect. Sometimes, people confuse the term asterism with constellation.

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