Submarines in the Winter Twilight

One of the more celebrated North Pole surfacings of the USS Skate happened today in 1959, see http://wattsupwiththat.com/2009/04/26/ice-at-the-north-pole-in-1958-not-so-thick/ for more on that and several others.

Nearly a couple years ago at the ICCC in Chicago, Lord Monckton noted that at the time of this surfacing it was late winter and the Sun hadn’t risen. I’m not entirely sure why he mentioned that, but I wasn’t able to come up with a quick and accurate description of the lighting conditions. Let me do it now so I can get the issue out of my system. First, we need a submarine:

I’m not about to spend my St. Patrick’s Day at the North Pole in hopes that a sub will surface, especially since there’s a perfectly accessible submarine in Portsmouth New Hampshire at Albacore Park. I go to an annual event in Portsmouth each February, which fit quite nicely into seeking photos set in an icy, snowy setting.

The rocket science part is to time the photo to match the Sun’s position below the horizon. At the North Pole the Sun rises in March and sets in September. The concept of a 24 hour day refers mainly to the longitude line the Sun is passing (it’s due south all “day” long).

Sunrise, by US Naval Office definition, is when the upper limb of the Sun touches the horizon. The USNO considers the horizon to be distant and the observer is close to land or sea surface. It’s fairly easy to compute the location of the center of the Sun relative the horizon as seen from the North Pole, it’s just the “declination,” which is the celestial equivalent of latitude. There are two significant effects to take into account. The obvious one is the Sun’s angular radius, which is about 16 arcminutes. The other is atmospheric refraction near the horizon. This is what makes the Sun and Moon look squashed when they are on the horizon. The refraction right at the horizon is about 34 arcminutes. So, sunrise occurs when geometrically the center of the Sun is 50 arcminutes (0.83°) below the horizon.

An aside – on the equinoxes, night and day are supposed to be equal. Not true! At most latitudes the time from sunrise to sunset is about 12h10m then. Today, for me, is the true clock time equinox.

Where were we? Oh yes. Some data from the North Pole on 1959 Mar 17, in degrees above the horizon:

1959 North Pole, Latitude    90.00  Longitude    0.00

  Date   Decln

Mar 15   -2.32

Mar 16   -1.93

Mar 17   -1.53

Mar 18   -1.14

Mar 19   -0.74

Mar 20   -0.35

Mar 21    0.05

It would be nice to know the hour, but let’s not be that obsessive. What does this tell us? First, the Sun will rise around the 19th. Second, Spring begins when the declination is 0°, and that will be on the 21st. On the 17th, the entire Sun is below the horizon and the upper limb is 0.70° below the horizon. This puts us well into morning twilight. Twilight – what’s that? Let’s take a look from a North Pole point of view, and start when it’s just plain dark.

Another aside – at the start of winter at the North Pole, the Sun is 23.44° below the horizon. However, the Full Moon will be between some 18° to 28° above the horizon. Yes, it’s nighttime, but not completely dark all the time.

If we discount the Moon and clouds, the sky is dark at the start of Winter with only stars providing feeble light. In late January, the Sun climbs above 18° below the horizon and we enter Astronomical Twilight. This is a period where there is enough light from the Sun that it interferes with seeing faint objects in the sky, especially those nearest the rising Sun. In mid February the Sun reaches 12° and we enter Nautical Twilight. Brighter, but mariners can still easily see the stars used for navigation. By early March the Sun reaches 6°, now the navigational stars are fading from view and we’re in Civil Twilight and people can get around pretty well without extra light. From the above, we can tell Civil Twilight will last a couple weeks and then daylight begins.

Now, let’s shift our point of view to Portsmouth. I’m also shifting to the evening because I took these photos after sunset. Civil twilight here lasts about half an hour, which fits in well with both our experience and state motor vehicle law that requires headlights to be on by half an hour after sunset.

Another aside – twilight is longest at the Summer Solstice, shorter at the Winter Solstice, but shortest near the equinoxes. If there’s interest, I’ll go into more details in the comments.

My intent was to photograph the Albacore in lighting similar to what there was for the Skate, i.e. when the Sun was 0.70° below the horizon. The math for this is a bit trickier, and entails messing with declination, right ascension, latitude, longitude, and Solar altitude to get the date and time. A fine approximation uses just the date (I was there on 2010 Feb 20) and some of this data:

2010 Albacore Park, Latitude   43.08  Longitude   70.76

  Date  Rise    Set  Civil   Naut  Astro    Decln

Feb 21  6:32A  5:22P  0:29   1:02   1:36   -10.43

At sunset, the solar altitude is -0.83°, we want the time when it’s about -1.53°. We know during Civil twilight the Sun will drop 5.16&deg, so interpolating, that’s about 4 minutes after sunset, and gee, there should be plenty of light. While the the visitor center and Albacore were locked, I could park there and walk around the trench used to bring in the Albacore. And indeed, there was plenty of light, as the photos prove.

Just one other thing – yes, black and white films in the late 1950s were plenty fast enough. The photographer probably used Tri-X (ASA 400) or Royal-X (ASA 1200). The latter was so grainy that it generally was only used in large format cameras. I had my camera set to a ASA 400 (or maybe even 100) equivalent.

Lord Monckton has probably figured all this out, but now we all know that there was plenty light and we know where to find a submarine in a snowy environment.

The Albacore is an interesting vessel, several design experiments with it influenced the next generation of submarines.