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Astronomy
Links Astronomical Observing and Sky ChartsThe night sky reveals a wondrous array of stars, planets, galaxies and gas clouds, many observable to the naked eye, or with binoculars. The incredible magnitude of deep space does not become apparent until viewed through a good quality telescope. There are several types of astronomical objects observable through the optical telescope. The most immediately apparent upon looking at the night sky are the stars, planets and moons. The planets and their moons are, of course, part of our own solar system, Sol. There are many millions of stars with their own solar systems, but only their stars are visible to us from Earth. Planets have been indirectly detected orbiting some of these stars by some of our largest telescopes. Formation of stars begins in stellar nurseries that are huge clouds of gas and matter. These clouds of gas, or nebulae, form some of the most prominent objects in space. They provide interesting viewing through telescopes. We, of course, live in the Milky Way Galaxy. Our own galaxy stretches across the sky in a wide band, containing millions of stars, visible as a radiating faint glow of light. There are many other galaxies, each containing millions of stars. Most of these are only visible with powerful telescopes and long exposure photography. Most easily visible of the galaxies are the Large and Small Magellanic Clouds, the closest galaxies to our own. They are, in fact, satellite galaxies on the edge of our own Milky Way. Most abundant of stellar objects are clusters of stars. Open Clusters are loose clusters of 10 to 1000’s of stars, usually in some distinctive arrangement. Globular clusters are dense clusters of 1000’s to tens of 1000’s of stars, usually in a spherical arrangement. Omega Centauri and Tucanae are the brightest globular clusters. Pleiades, or the Seven Sisters, the Beehive Cluster and the Jewel Box are some well known Open Clusters. There are several factors that affect our ability to observe the night sky. These are: (1) our own atmosphere – turbulence and light scattering diminish our views. (2) light pollution – our single worst enemy close to cities and towns (3) seeing – clarity of the sky directly overhead – mostly due to pollution (smog) and weather variations. Most clear nights, forecast with fine weather, produce good seeing conditions. In cities, beware of bright lights and periods of high pollution (smog). In summer, rising air currents, following a hot summer day, can produce poor seeing. Avoid windy conditions and moisture laden air. This is why many amateur astronomers travel to dark sky sites, away from cities and towns, to do their observing. Our most important observatories are located at high altitude to escape light pollution and atmospheric turbulence. Reading a Star ChartA Star Chart shows the position of celestial objects for a given time, date and location on the earth. The position of stars, nebula and galaxies is fixed in space, so they remain in the same position in the sky at the same time every year. The positions of the planets change every year. To use a sky chart, select the time, date and location of your observing site. Hold it above your head with north pointing to your north. Identify a prominent star on the chart and locate the same star in the sky. The positions should roughly match. Planetarium software automatically draws sky charts for any time, date and location. They also provide different views of the sky. A typical sky chart view shows the zenith directly overhead. A local horizon view shows the section of sky directly above your local horizon.
Top Astronomical Objects for Amateur ObservingSouthern Hemisphere
Northern Hemisphere
NomenclatureAstronomers have developed a way to map the night sky very accurately. Star positions are referenced using coordinates in a similar manner to longitude and latitude on ground maps. The longitude and latitude are replaced with right ascension and declination. Declination is simply the angle north or south of the celestial equator in degrees. Right ascension is the angle measured eastwards from where the ecliptic intersects the celestial equator. Another coordinate system, using altitude and azimuth is also used. Altitude is the angle above the horizon in degrees. Azimuth is the angle form true north, in degrees. The alt/az system is a local system and changes as the star moves across the sky. The RA/DEC system is a global system and fixes a stars position, permanently giving it RA/DEC coordinates, ignoring its apparent motion across the sky. The RA/DEC system is the standard coordinate system for measuring the position of celestial objects. Zenith - The zenith is the point directly overhead. Alt 90 degrees. Ecliptic - The ecliptic is the line of apparent motion of the sun against the stars during 1 year. Celestial Pole - The celestial pole is the point at which the axis of the Earth’s rotation intersects the celestial sphere. The celestial pole is at DEC 90 degrees. Celestial Equator - The celestial equator is the plane at right angle to the celestial poles (Dec 0 degrees). Celestial Sphere- The sphere directly overhead on which the stars lie. Equinox - One of the two points where the ecliptic intersects the celestial equator is the equinox and is also RA 0 degrees.
A typical Sky Chart showing the North Celestial Pole, Alt/Az grid, zenith and ecliptic.
A typical Sky Chart showing the North Celestial Pole, RA/DEC grid, zenith, ecliptic and equinox.
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