MAX EBB — "D
arn it," I thought to myself, "missed it again." It happens twice a year, and once again I had failed to sign up in time. The yacht club lawn was dotted with a dozen navigation students, each with a cheap plastic sextant, each struggling to establish the latitude and longitude of said yacht club lawn. I always intend to sign up for these one-day celestial navigation workshops, but never seem to find out about them in time. That's what happens when you don't read the club newsletter all the way through. But this time, as I drove into the parking lot, I noticed something different. The students were not on the south lawn,
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bar, just as the navigation class was coming in from the north entrance, notebooks and strange contraptions in hand. Lee Helm, who was apparently serving as the instructor this year, was herding the class back inside from the rear. Lee read my mind. The balloon over my head read, "What in heck are those things?" so I didn't have to say a word. "You'll never guess," she grinned. "Some kind of range finder?" I ventured. But there were no mirrors and no lenses. Just two long arms connected at a single Why would you use a rotating joint, and a protractorlike scale in the middle for reading backstaff instead of a the angle between the two arms. sextant? Why use a sextant "A two-arm version of a threearm protractor?" I guessed again, when we have GPS? even though the numbers on the scale did not seem to correctly indicate the angle between the but on the north side of the clubhouse, two arms. and they were facing north, not south. I "Getting colder," Lee answered. checked my watch: 13:15, high noon in "I give up," I conceded. But I daylight saving time. knew it was too soon for Lee to I know enough celestial to know that give up the secret. I found a barthey should be facing south, not north, stool off to the side of the group so for the noon sight. And they weren't I could eavesdrop on Lee's lecture. using the plastic sextants that the club "The backstaff," she began, keeps on hand for training — it was some "was invented by John Davis in other gadget that I could not name. 1590. It was, like, an order-of After mooring the car in one of the -magnitude improvement over the parking-lot side ties, I walked into the cross staff. Because with the cross staff, you Étoile had to look in two directions at the same time, and even though most people have two eyes, you really can't line up two objects in different directions at the same time. Especially for he sun sights when the angle c é l F at noon is high, like today. Also, they didn't have sunglasses in those days, and looking right into the sun Horizon is bad. I conclude that the cross staff was used much more effectively at night, for measuring the altitude of stars at lower angles during meridian transit, which is, like, a noon sight for a star." "We have the table of declination for the sun for The cross staff. This instrument required looking in two direc- every day of the year," said tions at the same time, and one of those directions was directly one of the students. "Do you at the sun. It was probably used more often for meridian transits also need a table like that for of stars at low angles, but required moonlight to see the horizon. each star or planet?" u
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For the planets, for sure," said Lee. "But not for the stars. They are far enough away so we call them 'fixed' with respect to the celestial sphere. Their geographic positions — that's the point on earth from which the star appears directly overhead — goes around the earth every day. But, like, the latitude of that point never changes. In the jargon, that's the same as saying that the declination of the star is constant. "So star shots are simple, no astronomical tables needed except a short list of the declination of each star, good for all years and all dates. Measure the altitude of a star at the star's 'noon' or 'midnight,' and after a little arithmetic you have your latitude." "I can get how a star has a 'noon' when the longitude of the geographic position is the same as our longitude," said another student. "Just like a noon shot of the sun. But midnight? How can you see the star at the star's midnight?" "Think of a star close to Polaris," Lee answered. "It appears to circle around