Adrian Dening's Stars Over Somerset

Thursday, 13 March 2025 13:20

Monday 17th to Sunday 23rd March 2025

We're going to stay close to home next week because Thursday 20th is the vernal or spring equinox. What on Earth does that mean?

Our planet is tilted by 23.5 degrees or in other words, the north and south poles are not directly at the top and bottom. If you've ever seen the classic globe in a classroom, you will notice that it is not mounted vertically on its base, but 23.5 degrees off-centre.

We take 23 hours 56 minutes and 4 seconds to rotate once on our axis and this is one Earth day, but it is rounded up to 24 hours to keep things simple. It takes the Earth a year, 365 1/4 days, to orbit around the Sun, but again we keep it simple by calling a year 365 days precisely and every four years, take the extra quarters to create an extra day in February - a Leap Year.

As we orbit around the Sun, at one point our 23.5 degree tilt is angled towards the Sun and we have summer in the Northern Hemisphere. The Sun appears a little higher in the sky as a result and the daylight hours last longer. Six months later, the tilt is pointed away from the Sun and it is then winter in the Northern Hemisphere, with the Sun lower in the sky and the daylight hours correspondingly shorter. Half-way between each of these points, our tilt is sideways-on to the Sun and we have periods of equal day and night - the spring and autumn equinoxes.

This explains why, when we have summer, the Southern Hemisphere has winter and vice versa, but what about the missing minutes each day where we have rounded up to 24 hours? It's why the days get longer and shorter every day by approximately four minutes as we go through the seasons!