by Tony Alegria
Although we are circling the Sun at some 108 000 kilometres per hour (kph) giving us our years and rotating at 1 800 kph to give us our days, we have no sensation of speed. In fact, it feels like we are standing still and everything in the sky is moving from east to west. That’s what our forefathers believed – no wonder they thought that the Earth was at the centre of the universe! They saw all the stars, the sun and the moon all moving from east to west and noticed that a few bright stars moved quicker than most and called these stars “planets” – from ancient Greek meaning wandering stars.
They associated the wandering stars with gods and hence named them after their Greek Gods. They also noticed that the sun followed a path overhead nowadays called the ecliptic. Eight degrees on either side of the ecliptic is an area called the Zodiac in which the moon and planets move along. The constellations on the zodiac are those used in Horoscopes.
Now, when a planet, moon or sun appears to be within the boundaries of any of these constellations, it is said to be “IN” that particular constellation e.g. on 15th September 2018 the naked eye planets: Venus will be in Virgo; Jupiter in Libra; Saturn in Sagittarius and Mars in Capricorn.
However, the moon, stars and planets all appear to be equidistant from us, as if they were all on a three dimensional screen. This screen is called the Celestial Sphere and is depicted in the diagram below by the dark blue circle – Earth is at its centre.
CyberSky describes the Celestial Sphere as an imaginary sphere on which celestial objects appear to be located. As the Earth rotates on its axis from west to east, the celestial sphere appears to rotate in the opposite direction, from east to west.
If the Earth’s North/South axis (the line through the North and South Poles) is extended to touch the Celestial Sphere, the North Celestial Pole (NCP) is represented by Polaris (the North Star) but the South Celestial Pole (SCP) has no suitable South Star. The constellations near the NCP and SCP will appear to rotate around these points.
Our forefathers believed that Earth was at the centre of the universe and that everything went around Earth, but Venus was a mystery. It would appear in the evening in the west, get higher and higher by the day, reach a maximum point and then it would get lower and lower until it disappeared below the horizon. Later on, it would pop up in the East before sunrise and do the same thing. Then later on it would again show up in the west! The period between Venus being the Morning Star and Evening Star is about five times longer than that between being the Evening Star and Morning Star.
By Aristotle’s time, the Venus mystery had been solved and the universe was believed to be as per diagram below with Earth still at the centre – the geocentric model. The Sphere of the Prime Mover was the force which moved all the fixed stars, the “wandering” stars, the moon and the sun.
The Astronomy Society of Harare has a presentation at 6pm on the second Wednesday of every month (except for December and school holidays) at the Lecture Theatre at St. Georges College. Non-members welcome. You don’t need to be a boffin or own a telescope to come along to these meetings – you just need an interest! If you have any questions regarding the Astronomy Society or about this article, kindly contact Tony Alegria on: tonyalegria47@gmail.com