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The Moon and Spring Tides

During the winter, the days become shorter. The mornings are darker and the Sun sets earlier in the evening. The amount of sunlight that we get each day becomes less and less, until finally we reach the shortest day of the year. This day is known as the winter solstice, and in 2017 it will fall on 21st December – in the northern hemisphere, at least. (If you look at a world map, the northern hemisphere is the top half, while the southern hemisphere is the bottom half.) When it’s winter in the northern hemisphere, it’s summer in the southern hemisphere. How on Earth does this work?

You probably know that the Earth is constantly spinning, and that this action is what gives us daytime and night-time: the side of the Earth facing the Sun experiences daytime, and the side of the Earth facing away from the Sun experiences night-time. But did you also know that the Earth spins – or rotates – at a tilt? This is what gives us seasons.

The tilt means that as the Earth orbits (moves around) the Sun, one hemisphere receives more light and heat than the other. The hemisphere that is pointed away from the Sun experiences winter because it doesn’t receive much heat or light. The hemisphere that is pointed towards the Sun experiences summer, because it gets lots of heat and sunlight.

Our winter solstice occurs on the day that the largest proportion of the northern hemisphere is tilted away from the Sun. Of course, this means that the largest proportion of the southern hemisphere is tilted towards the Sun – which means that this is when their summer solstice happens!

For many people these days, the most exciting thing about the winter solstice is that it means that the days will finally start getting longer. However, this time of year has actually been important to humans for thousands of years. The winter solstice was celebrated every year by many ancient cultures, as it helped farmers keep track of when they should start planting their crops again for the coming year. It also gave hunters a good idea of when to go hunting for animals that only appeared in the area once a year, like migrating salmon.

In preparation for the coming cold and harsh winter months, when food was scarcest, farmers would kill some of their farm animals so that they would have fewer mouths to feed. This meant that winter was one of the only times of year that people were able to eat lots of fresh meat.

So how did human populations, thousands of years ago, know when to plan their winter solstice celebrations? They watched the horizon carefully over several years, and noticed that the Sun rose and set in different places each day throughout the year. They then built stone structures that would reflect the Sun’s light in beautiful ways at different times of year. In a way, these structures functioned a bit like a huge clock that showed the seasons. Let’s look at an example.

One of the most famous is Stonehenge, England. Stonehenge was built roughly 5000 years ago from huge stone blocks that were arranged in large circles. On the morning of the winter solstice, the Sun rises and shines on the smooth side of the tallest block of stone. That evening, the Sun sets neatly between two other large stones in the stone circle. It never rises and sets in exactly this way at any other time, so when this happens, you know you’ve reached the shortest day of the year – and for the next six months, the days will slowly become longer again.

Watch The Moon and Spring Tides to see how the seasons can affect tides!