So today, 21st June, marks the summer solstice. The longest day of the year and the midpoint of summer.
If you were to mark out the position of the sun on the sky at midday every day for a year, the summer solstice is the day when the Sun is at its highest point in the sky. Conversely, the winter solstice (21st December) is the day when the Sun is at the lowest point of the year at midday. This naturally leads to the solstices being the longest and shortest days (or shortest/longest nights) of the year, as in summer, the sun has longer to travel on the sky from sunrise to sunset, and shorter in winter.
The reason we have the solstices is for the same reason we have seasons, the Earth is tilted on its axis. In the summer, in whatever hemisphere you are in, the Earth is tilted towards the Sun, in the winter, it is tilted away from the Sun. The summer solstice marks the point in the Earth’s orbit when the axis is most tilted towards the Sun, and when it is the summer solstice in the northern hemisphere, it’s the winter solstice in the southern hemisphere.
Now, you probably already know about the summer and winter solstices, but not everyone has heard of the vernal or autumnal equinoxes (around March 21st and September 22nd), . These are analagous to the solstices, in that they mark the midpoint of spring and autumn (or fall) respectively. However, the equinoxes mark the time when day and night are of the same length, which corresponds to the perpendicular points on the Earth’s orbit to the solstices.
The equinoxes are formally recognised when the position of the Sun on the sky crosses the Celestial Equator, which is the projection on the sky of the Earth’s equator.
With a bit of simple stellar geometry it is quite simple to prove all of this stuff, but I’ll not bother here.
I’m sure someone out there finds this stuff interesting!
