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.

Two Sunsets Today in Sodankylä!

Two sunsets on the same day? For real?

This is something that never happens at midlatitudes, but which occurs once a year in places which are located far enough north (or south, for the southern hemisphere). This is the case today in Sodankylä, as advertised in the almanach published by the University of Helsinki.

The July pages of Yliopiston Almanakka 2015, by the University of Helsinki.

Of course, between those two sunsets today, we do have a sunrise. What happens is that, since 15th July, when the midnight sun period ended, the sun has been setting after midnight every day, for a short while below the horizon. This is due to the fact that Sodankylä is located at 26°35' east longitude, which means that local solar midnight happens at 22:14 UT (since 15° in longitude corresponds to one hour). As Finland is now in summer time (i.e., UT+3:00), the solar midnight in Sodankylä therefore occurs at 01:14 LT. In other words, sunrise and sunset are on average symmetric with respect to 01:14 LT (00:14 LT in winter time)¹. There is therefore a period during which, every day, sunset occurs after midnight, and thus practically before sunrise. After the end of the midnight sun period, the sunset time moves gradually from near 01:10 towards 00:00, until the day when it crosses this critical boundary.

And indeed, last night we had a first sunset at 00:03 local time, and the sun rose this morning at 02:38. It will set again tonight at 23:57, and that is why we have two sunsets today in Sodankylä.

Plot of the solar elevation on 15 July 2015 in Sodankylä. The horizon line (in red) is shown slightly below 0° elevation since the sun is considered set when its upper limb is below the horizon (the blue curve refers to the centre of the sun).
Figure made by T. Ulich.

In a similar way – and because there must be the same number of sunrises and sunsets over a complete year –, there is a day when there is no sunset at all. Indeed, before the midnight sun period starts, sunset occurs later and later in the evening, until it ultimately moves from typically ~23:56 to ~00:03, skipping a day in between. This happened on 24 May this year in Sodankylä.

Interestingly, one may notice that these funny sunset patterns are not limited to latitudes beyond the polar circle, and not even to midnight-sun latitudes². The almanach indicates that there were two sunsets on 6 July in Oulu, although Oulu is too far south to be in the midnight-sun area. This can be explained by the fact that there is no need for a midnight sun period for this phenomenon to occur; all that is needed is nights which are short enough to have the sunset taking place after midnight at least one day. The further west in the time zone, the more south it is possible to go to find a suitable place, because the local solar midnight is shifted later with respect to the official local time. Empirically, I could find that the southernmost populated place in Finland where two sunsets can be observed on the same day would be Rahja (500 inhabitants, about 15 km south from Kalajoki). Perhaps they should advertise it on the official webpage of the village (Rahjankylä); there might be potential to attract tourists.

A final question to conclude: is it possible then to have two sunrises on the same day? If so, when and where? I will send a postcard to the first person who gives the correct answer in a comment to this article or to the associated Facebook post on the SGO page ;)

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¹In practice, the exact local solar midnight varies throughout the year due to the ellipticity of the Earth's orbit. It follows the so-called equation of time. These days, in Sodankylä, the actual local solar midnight is at 01:20 LT.
²Those extend about 0°50' below the polar circles, because of the combined influences of the apparent width of the sun (see the plot caption) and of atmospheric refraction.