June 21 marks the official "first day" of summer, or in more technical terms, the summer solstice.
A solstice is an astronomical event that happens twice a year (like equinoxes) when the Sun's apparent position in the sky reaches it's northernmost or southernmost extremes.
The name is derived from the Latin sol (sun) and sistere (to stand still) because at the solstices the Sun stands in declination; which means that the apparent north and south movement of the Sun comes to a stop before reversing direction.
The terms 'summer solstice' and 'winter solstice' are the most common names for this phenomenon, although they are somewhat ambiguous because the seasons of the northern hemisphere and southern hemisphere are opposites; the summer solstice of one hemisphere is the winter solstice of the other.
Which is why here in North America it is summer, but in Australia, they are starting their winter. Following me so far??
Very interesting stuff....yes. I've got you all riveted, I can tell.
At any rate, the cause of the seasons is that the Earth's rotation on it's axis is not perpendicular to it's orbital plane but currently makes an angle of 23.44 degrees, and that it keeps it's orientation with respect to inertial space. Consequently, for half the year the northern hemisphere is inclined towards the sun, with the maximum inclination occurring at about June 21 (the longest day of the year!) and for the other half of the year, the southern half of the hemisphere has this distinction, with the maximum around December 21 (the shortest day of the year....booooo!).
Whew. It's hard work spouting off all that scientific blah-de-blah. I had to look some of that stuff up. Seventh grade science class was many eons ago for me. LOL!
I hope all you Northern Hemis (I made up that word) have a nice first day of summer!
Thanks for stopping by!