Summer is here, the date varies between June 20 and June 22, depending on the year, and which time zone you are in. If you live in the U.S., the first day of summer will be June 20. In Europe, Asia, Australia and parts of Africa it will fall on June 21. When the Sun is at its most northerly point, hovering over the Tropic of Cancer, that’s known as the summer solstice.
The solstice occurs twice in each circuit of the earth around the sun. A circuit of the sun is how we define a year, so there are two solstices a year, winter and summer. The winter solstice is in late December and the summer solstice in late June in the Northern Hemisphere. The word solstice is formed from two Latin words; one for “sun” sol and one for “stand” sistere. The summer solstice is when the sun appears to be at its greatest and the day is at its longest. The winter solstice is when there is the least amount of sun. The other two seasons, fall and spring, are marked by the equinoxes when day and night are roughly equal.
The science is fairly straightforward: The Earth’s axis is tilted by about 23.5 degrees from its orbital plane. In other words, our planet spins around its axis like a spinning top, but the North Pole is always “off vertical” by 23.5 degrees.
As the Earth orbits the sun, the North Pole is pointing preferentially toward the sun during the summer in the Northern Hemisphere (now), but points away six months later on Dec. 20 or 21. For the northern hemisphere, when the North Pole is tilted toward the sun, it’s the summer solstice; when pointing away, it’s the winter solstice. For the Southern Hemisphere, the opposite is true (i.e. when it’s the summer solstice in Canada, it is the winter solstice in Australia).
Summer solstice is also a reminder about how lucky we are to be living on a planet with a tilt.
If the Earth spun vertically — with no tilt relative to its orbital plane around the sun — the equator would always have the sun directly overhead and the poles would be in perpetual twilight.
Every day would be a summer solstice for the equator and winter solstice for the poles. There would be no seasons, and the world as we know it would be a very different place: a lifeless desert around the equator and frozen poles equally as hostile to life.