When does summer actually start? Do you think summer starts as soon as school ends? Or, do you think it starts as soon as the sun is out and burns your skin? The last day of school, sunny days and sunburns are all signs of summer. But, in the northern hemisphere (the top half of the globe), summer does not actually start until June 21st.
To be exact, summer starts at 0148 UT on June 21st, which is 9:48 Eastern Standard Time (our time zone) on June 20th. Yes, that means summer actually starts during the night of the 20th for us. But, June 21st is still the first full day of summer. Known as the summer solstice (sun stands still), June 21st is the longest day of the year.
In the southern hemisphere, below the equator (the line in the middle of the globe that splits the world into two), winter arrives. For them, June 21st will be the shortest day of the year. It also marks the start of winter for them.
Why do the weather and seasons change? To answer this question we have to think about the planet we live on. The Earth takes a yearly trip around the Sun. For part of the year, the Earth's north pole points away from the Sun and part of the time toward it. When the North Pole points toward the Sun, the Sun's rays hit the northern half of the world more directly and it is summer. But when the North Pole is pointed toward the Sun, the South Pole is pointed away. So the Sun's light hits the Earth at a less direct angle, spreading the warmth over a larger area, and it is winter.
Some people think the seasons are caused by how far the Earth is from the Sun. But, the Earth's orbit about the Sun is very close to circular and the distance of the Earth from the Sun only differs by about 3% during the year. Another problem with this hypothesis is we are actually closest to the Sun on about January 2nd, and the farthest on about July 4th. This is the opposite of hot and cold weather in the northern hemisphere. Therefore, the angle at which sunlight hits the Earth is more of a cause of seasonal changes than the Sun's difference from Earth.
Another factor in seasonal changes is the length of days and nights. In the summer, daylight lasts longer and nighttime is shorter. This makes the temperature higher. In winter, the days are shorter and the nights longer. The winter gives the sun little time to warm up the Earth so short winter days do have long, cold nights.
Speaking of winter, the shortest day is the first day of winter. For us in the north, it is around December 21st or 22nd. This day is known as the winter solstice.
Between winter and summer, we have spring. It starts on about March 20th. On this day, day and night time are each 12 hours long. This is called the vernal equinox. It is the first day of spring north of the equator and the first day of autumn in the southern half of the world.
In between summer and winter there is another equinox, called the autumnal equinox. Just like the vernal equinox, day and night each are 12 hours long. But, it is now the first day of autumn north of the equator and the start of spring to the south.