Introduction: How do you mark the first day of summer? Is it the first hot day after a rainy spring? The last day of school? Perhaps it's the first outdoor party of the year with family and friends. 

Actually, the first day of summer is the day with the most hours of daylight due to one of Earth's poles pointing most toward the Sun. This is called the summer solstice and it occurs around June 21 in the Northern Hemisphere. Earth is tilted at an angle of about [math]23.5deg[/math]. As it orbits the Sun throughout the year, Earth remains tilted. When Earth is at the point in its orbit where the North Pole is tilted most toward the Sun, the Northern Hemisphere experiences the first day of summer. On this day the Sun's rays shine most directly on the Northern Hemisphere. It's important to understand that the seasons are caused by the tilt of Earth's axis, not the distance from the Sun.

But, what about the Southern Hemisphere? While the Northern Hemisphere is experiencing the summer solstice, the Southern Hemisphere is tilted away from the Sun. Since Earth's South Pole is tilted at its farthest point from the Sun, it is the first day of winter in the Southern Hemisphere. The first day of winter is the winter solstice. In the Northern Hemisphere, the winter solstice occurs around December 21. So, when it is the winter solstice in the Northern Hemisphere, it is also the summer solstice in the Southern Hemisphere!

There are also two points during Earth's orbit of the Sun where we experience almost equal amounts of daytime and nighttime. This happens when the Sun seems to cross Earth's celestial equator. Can you guess when those occur? If you said spring and fall, you are correct! These are the equinoxes. The vernal equinox, also called the spring equinox, occurs around March 21 in the Northern Hemisphere. The autumnal equinox, also called the fall equinox, occurs around September 23 in the Northern Hemisphere. Just as with the solstices, if it is one equinox in the Northern Hemisphere, it is the opposite equinox in the Southern Hemisphere.

Now, try the practice questions to see what you know about solstices and equinoxes.