Northern Lights Season has started

The first northern lights of the season have been spotted as far north as Sodankylä last night. This has raised the question on when it is dark enough to see them. The northern lights are comparable in brightness to the stars: if you can see the stars, you would see also northern lights, even if the contrast against the bright sky is still very low.

In more definite terms, for the night to be dark enough, the Sun should be some 8°–9° below the horizon. For our scientific all-sky cameras, the limit is -10° solar elevation for at least a couple of hours. This means that it is turned off in mid-April, and could be deployed in autumn as early as late August, but in practice this depends on operational matters, since all cameras are recalibrated and serviced every autumn, which is a collaborative effort with colleagues from other institutes.

In order to give you an idea when the aurora season starts at various latitudes in Finland, we have computed the time of sunrise and sunset as well as the start and end of civil, nautical and astronomical twilight. These are plotted below in "length-of-day/night" diagrams (click to enlarge) for the locations of Utsjoki, Sodankylä, Oulu, Jyväskylä, and Helsinki. The green line in those plots refers to the -10° elevation limit of the auroral camera. These diagrams are explained in somewhat more detail in our article about the Polar Day.

Twilight refers to the time when the Sun is below the horizon, but it is not completely dark. Civil twilight refers to the Sun being between the horizon and -6° elevation. During civil twilight, observations of stars and aurora are possible only using special equipment. When the Sun is between 6° and 12° below the horizon, we speak of nautical twilight, and when the Sun is between 12° and 18° below the horizon we call it astronomical twilight. Once the Sun reaches -18° elevation (18° below horizon) we have complete darkness. Note that this does not happen in Utsjoki before mid-September. But even in Utsjoki, the northern lights season will start in about a week.

Happy aurora hunting!

Length of Day/Night at Utsjoki (click for larger image)

Length of Day/Night at Sodankylä (click for larger image)

Length of Day/Night at Oulu (click for larger image)

Length of Day/Night at Jyväskylä (click for larger image)

Length of Day/Night at Helsinki (click for larger image)

See also: Polar Day.

Polar Day Ahead!

The nights are getting brighter, and we are rapidly approaching the Polar Day, when the Sun does not set anymore for many weeks. The All-Sky Camera of SGO requires that the Sun has set at least 10° below the horizon before it will start up to take images of the night sky and the aurora if there's any. On Friday, 24th April, the Sun will not set below –10° elevation, and thus the camera operations have now been stopped for the summer. The next time the Sun goes low enough in the night is on 19th August. Thus from our auroral camera's point of view, the Polar Day is about to start, and it will last for 117 days!

Please note that for operational reasons, SGO's All-Sky Camera will not be immediately available from 19th August onwards, but of course we try to make the real-time images available as soon as feasible.

The image above (click to get a larger version), shows the light conditions in 2015 for the location of our All-Sky Camera. On the horizontal axis there are key dates for this year. On the vertical axis are the number of hours of darkness. Yellow refers to daytime, i.e. the Sun is above the horizon, and in the middle of the year, there are some 44 days when the Sun does not set at all here. The darkest blue refers to the time when the Sun is 18° or more below the horizon. This is considered total darkness.

The remaining shades of blue refer to the twilight: the lightest blue represents civil twilight, when the Sun is between 0° and 6° below the horizon, i.e. the time just before sunrise or after sunset. The next darker blue with the green line refers to nautical twilight when the Sun is between 6° and 12° below the horizon. The green line shows the limit for the All-Sky Camera, when the Sun is 10° below the horizon. The remaining shade of blue refers to astronomical twilight when the Sun is between 12° and 18° below the horizon.

The green line goes to zero (no darkness anymore) on 24th April and rises again on 19th August. In the middle of winter, the Sun remains for more than 16 hours low enough for our camera to operate.

Furthermore, we can see that just around winter solstice, the Sun does not rise at all during four days. However, it is not totally dark thanks to the prolonged civil twilight time and usually bright, snow-covered land.

Graphic and solar data: Thomas Ulich.