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. In 2012 the summer solstice is on 20 June at 23:09 UTC.

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.

Figure 1: Meteosat-9 SEVIRI image of the northern solstice over the polar region, taken at midnight on 21 June 2011 (Click to enlarge)

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, below, 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'.

Figure 2: Solstices and equinoxes from 2011 and 2012. (Click to enlarge)

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