Summer Solstice

June 20th, 2016 marks the official beginning of summer in the northern hemisphere. We call this day the summer solstice.

In astronomical terms, the summer solstice for the northern hemisphere occurs when the sun is the farthest north of the equator on the ecliptic. This angular distance is 23.5°, This angle also marks a line of latitude on the Earth called the Tropic of Cancer.

The Tropic of Cancer is so called because the summer solstice used to occur when the Sun was in the constellation Cancer and the latitude lines in the tropical zone. There is also a comparable line of latitude south of the equator called the Antarctic Circle. The latitude is 66.5° South and for anyone living south of this latitude (really, mostly penguins or anyone stationed in Antarctica), the Sun will never rise. Conversely, the Arctic Circle at 66.5° North, the Sun never sets. In fact, from the vernal equinox to the autumnal equinox, the North Pole is in perpetual sunlight, while the South Pole is in perpetual darkness.

The summer solstice marks the day of the year when the northern hemisphere receives the most light. From here on until the winter solstice, the days will only get shorter and the nights longer. In the southern hemisphere, the opposite occurs. Today marks the shortest day of the year and they are in winter time.

Seasons on Earth

Recently, everyone on Earth went from one season to the next. In the northern hemisphere, we went from winter to spring and in the southern hemisphere, summer gave way to the fall (autumn). How exactly are the seasons defined?


Just this past Friday, the northern hemisphere experienced the vernal equinox, while in the southern hemisphere, the autumnal equinox occurred. We already know why they call these the equinoxes as they were explained in the previous posts. However, why does the northern hemisphere experience one season, while the southern hemisphere experiences the opposite (spring-fall and summer-winter)? It all has to do with the Earth's axial tilt.


The Earth rotates on its axis once a day. Most everyone knows that. However, the rotational axis of the Earth is tilted with respect to its orbital axis by about 23.5º.

So what does this mean?


When one half of the Earth is tilted towards the Sun, that half will receive more direct sunlight. The other half will not. It is like when you take a flashlight and aim it at a wall. If you keep the flashlight parallel to the wall, the light circle is tight and compact, if you tilt the flashlight away from the wall, the light circle becomes elongated. The same amount of light is hitting the wall, but in a larger area.

 

Another interesting thing about the seasons is that the time of year they occur change over time. However, none of us will be alive when January will be summertime in the northern hemisphere. This will occur in about 13,000 years. If you can wait until 28,500 CE (common era, formerly known as AD), April will be the middle of summer for the northern hemisphere. This is because the Earth wobbles on its axis, causing precession of the equinoxes (which in turn, also causes the solstices to precess). Currently, the summer solstice occurs when the Sun is in the constellation Gemini, but will someday be in the constellation Sagittarius.