The "A to Zee" of Astronomee.

By: Wolf Lange

Sources include: Collins Dictionary of Astronomy 2^nd Edition, Burnhams Celestial Handbook Revised and Enlarged Edition, Patterns in the Sky by Julian DW Staal and the Amateur Astronomers Handbook by JB Sedgwick.

Earth – The third "rock" from the sun. Largest of the inner planets with an equatorial radius of 6378km, it has one natural satellite, the moon. The earth has an atmosphere consisting mainly of nitrogen and oxygen. 2/3rds of the earth are covered by water. Ocean depth ranges from 2500 to 6500 meters and the average land elevation is 860 meters. The three main layers are crust, mantle and core with the crust 30 – 40km thick under the continents, much thicker under the mountains and only about an average of 6km under the oceans. The mantle extends to depths of about 2900km and the core is composed mainly of iron with many other components. Temperature at the core level will be in the region of 4000 degrees Celsius. Even though 4,6billion years old the internal heat of the earth is still a powerhouse that produces earthquakes, volcanic eruptions and moving of continents about its surface.

Earthshine – (ashen light) this is sunlight reflecting from earth and making the dark side of the moon close to new moon become faintly visible – the old moon in the new moon’s arms!

Eclipse - the total or partial obscuration of light from a celestial body as it passes through the shadow of another body. The popularly known events of the solar (sun) eclipse and lunar (moon) eclipse occur when sun, moon and earth lie in or nearly in a straight line. Although the moon is 400 x smaller than the sun, it is also about 400 x nearer to the earth. The result is that sun and moon have almost exactly the same angular size of about ½ degree, which makes it possible for the moon to totally obscure the sun. For us to observe a solar eclipse the moon will have to pass directly in front of the sun. The shadow this event causes will move in a west to easterly direction in a very narrow curved strip across the earth. Maximum totality duration is 7mins 31secs (normally this tends to be in the region of 1 to 3mins).

Ecliptic and ecliptic co-ordinate system – the mean plane of the Earth’s orbit around the sun. It may thus be taken as coincidental with the sun’s apparent annual path across the sky. The planes of the ecliptic equator are inclined at an angle equal to the tilt of the earth’s axis. This angle is known as the obliquity of the ecliptic and it is roughly 23,5 degrees. The ecliptic co-ordinate system refers to the ecliptic as the fundamental reference circle - broadly the circle along which apparently the sun and the moon and all the planets move. The zero point is the vernal Equinox. The co-ordinates are the celestial (or ecliptic) latitude and longitude. The ecliptic system is the older but less used than the equatorial and horizontal co-ordinate systems.

Elongation – the angular distance between the sun and a planet i.e. the angle being sun-earth-planet, measured from 0 to 180 degrees east or west of the sun. It can also be the angular distance between a planet and one of its satellites. An elongation of 0 degrees is referred to as conjunction and one of 180 degrees is opposition.

Emission Nebula – there are generally three types of nebula: dark, emission and reflection. Emission is the result of a nebula being associated with a region of hot interstellar gas and dust that (because of the high temperature) shines by its own light. The emission nebulae again group into three classes H II (ionised hydrogen) e.g. Orion nebula, UV or ultraviolet ionised gas and O and B stars nearby radiating and causing the nebula to glow. The spectacular colours are predominantly green and red and result from recombination and excitation (red hydrogen alpha and oxygen providing the green colour.

Encke’s comet - the most observed comet, having been seen at 55 apparitions. With a period of 3.3 years – one of the shortest – it is thought to be the parent body that gave birth to the Taurid Meteoroid Stream. The period of Encke decreases by up to 2.7 hours per orbital revolution. This is caused by a jet effect from its rotating nucleus, which is not thermally symmetrical. The brightness of Encke has hardly decreased during 165 years of observations. It will be a prime target for future spacecraft investigations, as its orbit is extremely accurately known.

Ephemeris – a work published annually in which the daily predicted positions of the sun, moon and planets etc are tabulated. Together with information about certain stars, eclipse etc it is used as a reference document for astronomical observations.

Epoch – an arbitrary fixed date or point of time that is used as a reference datum, especially for stellar co-ordinates and orbit elements. As the co-ordinates of right ascension and declination are constantly changing primarily as a result of precession of the equinoxes. Since 1984 the Julian year has been used, the current standard epoch, J2000.0 is 2000 Jan.1.5; it is exactly one Julian century removed from the standard epoch 1900 Jan 0.5. A standard epoch is usually retained for 50 years. (This means in layman’s language is that any star charts you may have in books or as individually published sets marked 1950.0 are out of date and if marked epoch 2000.0 you are assured many more years of usage.

Equatorial co-ordinate system – the most widely used astronomical co-ordinate system in which the fundamental reference circle is the celestial equator and the zero point is, strictly, the vernal equinox. The co-ordinates are right ascension and declination, which are measured along directions of terrestrial longitude and latitude. Sidereal hour angle is sometimes used instead of right ascension and south or north polar distance for declination. Presently the standard epoch in use is 2000.0.

Equatorial mounting – a telescope mounting in which one axis (the polar axis) is parallel to the earth’s axis while the second one (declination) is at right angles to it. The net result is that your telescope moves around the one axis making it (in theory) simpler to keep an object in sight) in practise with standard Newtonian telescopes this is not always so easy to achieve accurately.

Equinoxes – the two points on the celestial sphere where the Ecliptic intersects the Celestial equator. Simply the two points where the sun annually crosses the equator in its apparent annual motion. These are referred to as the spring and autumn equinoxes depending on where you are (southern or northern hemisphere and they occur usually around 22/23 March and 22/23 September. The origin is Latin and it literally means Equal Night meaning day and night is of equal length. Currently the northern autumn equinox lies in the constellation of Virgo.

Eridanus – (river) a long straggling constellation in the Southern Hemisphere extending from Orion towards the southern pole region. Except for zero magnitude Achernar the brightest stars reach only 2 or 3 magnitude. Not spectacular at all it forms a long string with one fine double star Acamar (Theta). RA 1.5 to 6h, dec 0 to –58 degrees area 1138 sq. deg.

Wolf Lange