According to NASA, it is not the Sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and its nearest star. The axis of the Earth is tilted 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.
But how can it be called the First Day of Summer and Midsummer at the same time?
The difference lies in the definitions created by culture, agriculture and astronomy. According to NASA, June 21st marks the beginning of Summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere.
The official start and end of summer can vary by country – not because summer starts a week or so earlier in one country than another in the same hemisphere, but because the recognition of the start of summer is often influenced by historical or cultural reasons particular to that country. Most countries recognize summer as starting on dates ranging in May and ending sometime in August.
Although the June Solstice marks the beginning of Northern summer, it is often called Midsummer. In traditional Gaelic culture the Summer Solstice represented the mid-point between the commencement of the Celtic Summer on May 1 and autumn on August 1.
Why does there seem to be such a lag time?
The 2013 Summer Solstice marks the highest exposure of the northern Atlantic and Pacific Oceans to the Sun’s heating radiation. The rise in ocean temperatures then provides the energy necessary to produce stronger hurricanes later in the year, usually around September.
But I thought June, July and August were the Summer months?
They can be if your culture or country recognizes those months as summer. In the U.S., those months have become associated with summer because school vacation (a.k.a Summer Break) has traditionally taken place during those months. But the choice of those months were influenced by the agriculture harvest seasons to allow children to help on the family farm and not due to the official beginning and end of the summer season.