Spark and Perl go looking
It is summer down there.
I can tell. Everything is green.
In the top half it is. Look at the bottom half.
That is winter.
At the same moment. On the same planet.
The Earth leans, and it keeps leaning the same way all year round.
Always towards the same far-off star — so for half the year our half is tipped towards the Sun, and for the other half away from it. In the north that star is Polaris, and the Earth’s axis — the line it spins around — points almost exactly at it.
Is there one in the south as well?
No bright one. Down there the axis points at a patch of sky with nothing much in it — the closest star to that spot is far too faint to steer by.
So in summer our half leans nearer to the Sun.
In June we are further away.
Further? But it is hot.
The Earth is five million kilometres further from the Sun than it was in January.
Leaning is not about getting closer. Lean towards the Sun and its light comes from high overhead. The same sunlight lands on a smaller patch of ground, so each bit of ground gets more light and more warmth. And the Sun shines for more hours. That is the whole of summer.
And it is the other way round in the south: their summer comes when the Earth is nearest the Sun. That adds only a little — the leaning is what matters.
Then there must be a day when neither half leans towards the Sun.
Two of them. In March and in September the line between night and day runs straight through both poles, and everywhere on Earth gets about twelve hours of each.
And the day we lean furthest towards the Sun?
That is the longest day of your year, in June. And the shortest day of theirs, on the very same afternoon.
In December it swaps: their longest, your shortest.
Push a stick into the ground and mark where the end of its shadow falls at midday. Do it once a week for a year.
The marks will draw you the whole thing, and you will not have needed a single number.
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