The June solstice is the Summer Solstice in the Northern Hemisphere and the Winter Solstice in the Southern Hemisphere. It took place at 15:54 UTC on 21 June.
24 January 2022
21 June 2019
By Jose Prieto and Vesa Nietosvaara (EUMETSAT)
Solstice marks the shortest night of the year in the northern hemisphere (NH). Above 66.6° latitude, the Sun is above the horizon 24 hours a day. Over the North Pole, the Sun describes a full circle at constant elevation for the duration of the day. That elevation decreases towards the horizon to reach it at the September equinox. For other polar locations the solar path is tilted and, in the course of the weeks goes under the horizon for a fraction of the 24 hours cycle.
Although the Natural RGB from 21 June at 12:12 UTC (Figure 1) has no special meaning in astronomical terms, by chance it shows a front near the western coast of South Africa as a herald of the starting of the winter season in the southern hemisphere.
Often around the solstice, the particular geometry between the Sun, satellites and reflective surfaces generates sunglint on satellite imagery. Figure 2 shows an area close to the Baffin Bay. GOES-16 through its 0.47µm VIS channel captured a strong sunglint due to grazing reflection from the Sun onto the satellite at 05:00 UTC on 21 June, a time close to midnight in most of the Americas.
Strong reflection occurs on flat surfaces, in this case mostly frozen sea waters. For any geostationary satellite, similar sunglint is present on midnight imagery at latitudes higher than 66.6° for the longitude of the geostationary satellite. In this example, the patches of sunglint show around 80°W, 74°N.
Another effect of the high Sun in the NH around the solstice are the parallax cloud shadows on satellite imagery. Figure 3 shows Meteosat (left) and Sentinel-3 (right) images a few days after the solstice on 27 June at 11:00 UTC, over central Spain.
Shades can be seen at the north-west cloud boundaries in the OLCI RGB, but at the southern boundaries in the Meteosat-11 image.
The dark boundaries are shaded pixels. Meteosat can see the ground in those pixels close to the southern cloud boundary, because it is located at a lower elevation than the Sun for that time.
However, the Sentinel-3 OLCI instrument, passing almost vertical above the cloud, at a higher elevation than the Sun, sees shadows on the north-west of the cloud. See Figure 4 for a practical explanation.
Other solstice cases
Cancer solstice 2020
The Cancer solstice, with the Sun directly over the Tropic of Cancer, took place in 2020 on 20 June at 21:43 UTC.
Annular solar eclipse on solstice
During the 2020 June solstice the shadow of the Moon, travelling from west to east, appeared in the images of three geostationary satellites.
December solstice near the southern polar circle
December solstice occurred in 2018 at 22:23 UTC on 21 December and could be 'seen' in satellite imagery.
Winter solstice 2017
As precisely forecast, the northern hemisphere winter solstice (summer solstice in the southern) started on 21 December at 16:27 UTC.