Today (20 June) is the solstice, the longest day in the Northern Hemisphere and the shortest in the Southern Hemisphere.
The date of the summer solstice depends on the shift of the calendar, in the Northern Hemisphere it can be any time between 20 June and 22 June.
For a period of some days around the solstice it is possible for our geostationary satellites to see what is known as the ‘midnight Sun’. Because of the position of the Sun, the sunlight is reflected off the Northern polar region and is seen by our Meteosat Second Generation satellites, as shown in Figure 1 (right).
At the same time as the summer solstice heralds the astronomical start of summer in the Northern Hemisphere, the winter solstice heralds the start of winter in the Southern Hemisphere.
This changing of the seasons takes place because the plane of Earth’s equator is tilted 23.5 degrees to our orbit around the Sun.
Figure 2 (right), shows the change in the Sun's illumination of the Earth due to the position of the Earth relative to the Sun.
The March and September equinoxes mark the times in the year when day and night are of equal length across the globe.
The December solstice marks the shortest winter day in the Northern Hemisphere and the longest summer day in the Southern Hemisphere.
The line that separates the portions of the Earth experiencing daylight from the portion experiencing darkness is known as the 'terminator line'.