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Last updated on April 23, 2014 at 1:22 EDT

Positional Astronomy

Positional Astronomy — Positional astronomy is the study of the positions of celestial objects. This is the oldest branch of astronomy and dates back to antiquity. Observations of celestial objects are important for religious and astrological purposes, as well as for timekeeping.

Ancient structures associated with positional astronomy include:

– Chichn Itz

– The Medicine Wheel

– The Pyramids

– Stonehenge

– The Temple of the Sun

The unaided human eye can detect about 6000 stars, of which about half are below the horizon at any one time.

On modern star charts, the celestial sphere is divided into 88 constellations. Every star lies within a constellation.

Constellations are useful for navigation. If you live in the northern hemisphere you can find north by locating the star Polaris. This star is always at a position nearly over the north pole.

Concepts

conjunction

A conjunction is the appearance of 2 or more planets close to each other on the celestial sphere. In Spring 2002, a rare grand conjunction occurred; in which Saturn, Jupiter, Mars, Venus and Mercury were all visible simultaneously in the west-northwest sky, shortly after sundown.

Inferior and superior conjunction

An inferior conjunction occurs when two planets lie on exactly the same side of the Sun, whereas a superior conjunction occurs when two planets lie on exactly opposite sides of the Sun.

Opposition
Should the Earth be part of an inferior conjunction with a superior planet, the superior planet can be said to be, “at opposition” to the Earth.

Transit

A transit of a planet is the event of the planet passing between the Earth and the Sun in such a way that the planet appears to move across the Sun’s disk. This can happen only with the “inner” planets, Mercury and Venus.

The transits of Venus are very rare, they currently happen in pairs, 8 years apart which repeat only after more than a century. The last transit of Venus was in 1882, the next three will occur on June 8 2004, June 6 2012 and December 11 2117.

In the 18th century, the transit pair of 1761 and 1769 were used to determine the value of the astronomical unit. The transits of Mercury, being closer to the Sun, are more frequent, about thirteen per century. The last one was on November 15 1999, the next ones will be on May 7 2003, November 8 2006, May 9 2016 and November 11 2019.

Inferior and superior planets

These terms were coined by Copernicus to distinguish a planet’s orbit’s size in relation to the earth.

Inferior planet refers to Mercury and Venus as they have orbits smaller than the Earth’s. He deduced this because the inferior planets were always seen near the Sun. Both inferior planets are seen in the west just after sunset or the east just sunrise.

Superior planet refers to Mars, Jupiter, and Saturn, Uranus, Neptune, and Pluto as they have orbits larger than that of Earth’s. He deduced this because the superior planets were often seen on the side of the celestial sphere opposite the Sun, thus implying that the Earth was between them and the Sun.

Copernicus was not aware of the existence of Uranus, Neptune, nor Pluto, as the telescope had not yet been invented.

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Positional Astronomy