21-04-2007 Revolving heavens (Article 82)

[Simon Kenny]

Revolving heavens (Article 82)

The heavens don’t revolve, it’s the other way round, our Earth is rotating. Even today, this supposedly simple fact goes unnoticed by a surprisingly large number of people. It’s easy to understand why. Just stand under the stars and watch the heavens rise and set over our heads. Even astronomy texts speak of celestial objects and their motions in the sky, so it can be confusing. Actually this is deliberate. It is easier to describe things as we see them for our immediate aspiration. Of course the real fact is that our planet turns on its axis approximately every 24 hours, causing the stars to rise in the east and set in the west. Our image shows the stars as trails caused by the spinning earth over a period of about 30 minutes, but which we see as revolving heavens.

Because our planet orbits the Sun, we are thus moving through space, and as a result any particular star rises approximately 4 minutes earlier each night. Hence it will rise another 4 minutes earlier the following night, and 2 hours earlier a month later. In a year’s time it will rise at the same time on the same day as we first observed it. As we travel around the Sun in our year, our night time sky offers us different constellations in different seasons, but over the year we see all those that can be seen from Ireland. We say this because there are a number of constellations that can only be seen from the Southern Hemisphere of Earth. Our Moon and the planets complicate things a bit as their paths move west to east across our sky. They still rise in the east, but if you watch carefully over a period — the Moon is easier as its movement eastwards is obvious even on a single night, - you will see that they do indeed move ‘against’ the sky’s apparent motion. As with any hobby, astronomy has its own confusing elements, but once you learn them you will enjoy it so much more.