Sunday, April 5, 2020

Stargazing for Mariners: Northern Sky

We have a new Regiment of the North Star that can be used for finding latitude from Polaris without the official USNO annual nautical almanac. It can be used anytime you see Polaris, on any date or year. The method relies on identifying one of two stars, Alkaid at the tip of the handle of the Big Dipper and Segin, the trailing star of Cassiopeia on the other side of Polaris.

Both stars are trailing stars, meaning as the sky rotates, these are the stern of the constellation thought of as a boat sailing around the pole, once a day. They are also about equal distance from the pole and opposite to each other.

They are not bright stars. Alkaid at is the brightest of the Big Dipper stars, which as a group represent typical Magnitude 2 stars.  The leading 3 stars of Cassiopeia are magnitude 2 and the trailing 3 are magnitude 3.  Segin is the weakest of the lot at 3.3.

Because these stars are faint, we have to rely a lot on the pattern of the constellation. Alkaid is very easy, at the tip of the handle of the Big Dipper, but we need more help with segin.

Cassiopeia looks like an "M" from the Middle... meaning standing at the pole of the sky where Polaris is. Keeping that in mind, you can find this group, located about the same distance from the Pole as Polaris is on the other side.

Also the trailing loop of the "M" Cassiopeia is lazy, meaning not as tall, which also helps find this weak member of the group.

Other tips that help are these:

(1) Often the entire sky between Cassiopeia and the Big Dipper is blank except for two stars, and they will be Polaris and Kochab, one the Guards on the tip of the cup of the Little Dipper.  Kochab, Polaris, and Alkaid are about the same brightness.

(2) The line between Segin and Alkaid passes very closely through the the pole of the sky, and if you zoomed in you would see that Polaris is on the Cassiopeia side.

(3) We are talking about a large span of the sky between Alkaid and Segin. At middle latitudes they span from about NW to NE in bearing.

(4) Since the stars are faint, when any part of either constellation is within some 10ยบ of the horizon, they are likely to be extinguished by the (clear weather) atmosphere.

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