In the Northern Hemisphere, summer is held to begin on the day of the Summer Solstice, the day (actually a moment in time) when the position of the sun reaches its most northerly limit before slowly beginning to move south again. The Summer Solstice also marks the longest day and shortest night of the entire year. (For dwellers in the Southern Hemisphere, the situation is reverse: the solstice which occurs in June is their Winter Solstice, and marks their longest night and shortest day of the year.)
In our modern Gregorian Calendar (which is a 16th century slight modification of the Julian Calendar, established way back in the time of Julius Caesar), the Summer Solstice falls, in any given year, on a date as early as June 20 and as late as June 22. The shifting of the sun’s apparent position throughout the year is caused not by any actual motion of the sun itself, but rather is the result of an effect created by the fact that earth’s axis is not perpendicular to the plane of its orbit around the sun but is tilted at an angle of a little more than 23 degrees. This means that, for one half (six months) of the earth’s annual journey around the sun, one hemisphere is pointed more directly at the sun than the other, and then they trade places at the Equinoxes. (In the Northern Hemisphere, the Equinox occurring in the month of March is called the Vernal Equinox, and the one occurring the month of September is called the Autumnal Equinox.) The hemisphere receiving more direct sunlight (and for a longer portion of each 24-hour day) experiences summer, and the hemisphere receiving less direct sunlight (and for a shorter portion of each 24-hour day) experiences winter.
Just how long the day is compared to the night on the Summer Solstice depends on your latitude (how far north or south you are on the earth’s globe, as measured from the equator). For those dwelling in the lower 48 states of the US, the daylight period on the Summer Solstice ranges from about 14 hours (southern states around the Gulf of Mexico) to about 16 hours (northern states close to the Canadian border). And for those living in the extreme north, the day is a very long one indeed: the sun is above the horizon for at least 24 hours on the Summer Solstice for any location within the Arctic Circle (about 66 degrees north latitude), and at the North Pole itself, the Summer Solstice marks the mid-point of a six month period during which the sun never sets below the horizon (though it never gets very high in the sky either)!
If you mark where the sun rises each morning as seen from exactly the same location during the course of a year, you will find that its rising points at Summer Solstice and Winter Solstice are separated (again, using latitudes for the lower 48 states of the US) by anywhere from about 50 to almost 70 degrees. So, for example, if you have live on a quarter acre lot that is oriented on a roughly east-to-west axis, from a given spot on your porch or from a given east-facing window of your house, you will probably see the sun rise from somewhere beyond the northeast corner of your yard on the Summer Solstice, and somewhere beyond the southeast corner on the Winter Solstice. If you mark the position carefully from one day to the next, you’ll also notice that the north-south movement is most rapid at the middle of the cycle (during the time around the equinoxes) and is slowest at either end (at the solstices). This is similar to the effect of tossing a ball into the air and catching it again: the ball is traveling fastest when it leaves you hand and just before it lands in your hand again, and as it reaches the top of its path, it slows, appears to stop and hover for a brief moment, and then slowly starts to fall, gaining speed as it descends. (The word solstice, in fact, comes from a Latin compound which means “stopped sun.”) This also means that while the Summer Solstice is indeed the longest day of the year, it is only a minute or so longer than the day before and the day after. In fact, the difference in length of daylight from one day to the next is only very slight for several weeks before and after the Solstice, and daylight hours run very long throughout the entire period. (The difference becomes more noticeable during the time surrounding the equinoxes.)
One commonly held (though false) assumption about the Summer Solstice is that it also marks the earliest sunrise and latest sunset of the year. This is not the case: due to the complicated nature of astromechanics, these events actually lag ahead of (earliest sunrise) or behind (latest sunset) the Summer Solstice by several days.
In ancient times, the Summer Solstice was regarded in many northern European cultures not as the beginning of summer, but rather as summer’s mid-point. For this reason, a coinciding date close to the Summer Solstice (June 24) has long been observed as Midsummer, and continues in some countries today (Sweden in particular).
