On Springtime (and National Security)

Today (March 20) is the first day of Spring, also known as the Vernal Equinox.  The Vernal Equinox actually has a specific astronomical definition: when the Sun, heading north, is directly over the Equator.

We can actually confine that to a specific moment of time; according to the United States Naval Observatory, Spring began at 11:02 UTC this morning, which translates to 7:02 AM EDT.
(They could give you the exact time down to the millisecond if you wanted it)

This raises a question: Why should the military, particularly the Navy, care about Astronomy, or time?

Imagine you’re on a boat in the middle of the Atlantic in the 1700s.  There’s nothing on the horizon; in fact, the last time you saw land was two weeks ago.  How do you know where you are?

One answer is to use the stars.  As it turns out, the stars in the sky seem to spin around a point in the sky – the North Celestial Pole (which is near, but not exactly at, Polaris).  The altitude of that pole happens to be the altitude you’re at: If you’re at the North Pole (90 degrees latitude), that circular point is directly overhead (90 degrees altitude).  If you’re in New York City, the pole will be 45 degrees above the horizon, because you’re at 45 degrees altitude.  And so on. (You’ll need to know if it’s North or South, but you can use the ship’s compass for that)

But what about longitude?  Well, we know the Sun moves East to West, and we know it takes 24 hours to do it.  360 degrees in 24 hours means that in one hour, it’ll be astronomical noon at locations 15 degrees to the West.  So, if you’ve got a clock on board set to Noon in Greenwich, England (aka Greenwich Mean Time), and it tells you it is 3 PM in England when it’s astronomical noon where you are, you know you’re at 45 degrees West Longitude.

Given that the ship’s clock was often the only safeguard against getting hopelessly, possibly fatally lost, the punishments for messing with it were severe. (Think Piratey-type punishments)

Nowadays, the Navy uses GPS satellites (which the Naval Observatory has a major part in), but the United States Navy always has at least one crewman aboard each ship who knows how to navigate by the stars, just in case they can’t use the GPS system (computer problems, enemy sabotage, massive solar flare frying the GPS satellites).

The United States Naval Observatory still publishes almanacs with the necessary information (http://aa.usno.navy.mil/publications/docs/almanacs.php), generally in collaboration with Her Majesty’s Nautical Almanac Office (Purveyors of Fine Azimuths and Altitudes to Her Majesty the Queen, http://astro.ukho.gov.uk/nao/services/).  Those almanacs are far more accurate than commonly available planetarium software like Stellarium, and take into account all kinds of minute details you don’t need or want to worry about.


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