Science Puzzle
When Is Earth Closest to the Sun?
Earth's orbit isn't quite a circle. Over a year our distance from the Sun swings between about 147 million and 152 million kilometres.
For someone living in the northern hemisphere, in Britain say, when in the year is Earth at its closest to the Sun?
The Answer
Early January. Earth is closest to the Sun, about 147.1 million kilometres away, in the first few days of January, right in the middle of the northern winter. It's farthest away, about 152.1 million kilometres, in early July.
So distance can't be what causes the seasons. If it were, the whole planet would have summer at the same time, yet when it's winter in London it's summer in Sydney.
The seasons come from Earth's tilt. Our spin axis leans over by about 23.4 degrees, and it keeps pointing the same way in space all year. In June the northern hemisphere is tipped towards the Sun. The Sun climbs high in the sky, its light strikes the ground more directly so each square metre gets more energy, and the days are long. In December the northern hemisphere is tipped away: the Sun stays low, its light arrives at a slant and spreads thinly, and the days are short. The southern hemisphere gets the opposite at the same moment.
The changing distance does make a small difference. Sunlight is about 7% stronger in January than in July, which slightly warms southern summers, though the southern hemisphere's huge oceans soak up much of the extra heat.
The principle: The seasons are caused by the tilt of Earth's axis, which changes how directly sunlight strikes each hemisphere and how long its days last. The distance from the Sun changes only slightly and isn't the cause.