Writer Needs Celestial Nav Help

[watha]

I'm writing a piece about locational technologies and want to include some very basic concepts of celestial navigation. Too bad I know so little about it. I'd like to talk to an expert who can walk me through it, via e-mail. Would anyone care to volunteer? Please contact me ASAP for more details. Thanks.

[DeepFrz]

hi ya watha! welcome to CF.

http://www.unlikelyboatbuilder.com/

This guy has just written a whole bunch on his blog. May be what you need.

Sorry, couldn't help myself...

[watha]

Hi yourself. Have I come to the right place for navigation info?

[DeepFrz]

Check my modified post.

[watha]

This looks marvelous. Thanks.

BTW, one very basic question: in the Northern Hemisphere isn't the sun always somewhere to your south?

[AquatiCat]

Hi Watha,

In the northern hemisphere north of 23.5 degrees N latitude, the sun is always somewhere to your south at noon. But if you think about, for example, being up within the Arctic circle, the Sun can be straight N, S, E, or W of you on June 21, depending on what time of day it is (the Sun makes a circle around your horizon if you are within 23.5 degrees of the pole, in the "summer" associated with whichever hemisphere you are in)! So, you need to be specific about time of day.

At Georgetown in the Bahamas (23.5 N), the Sun passes directly overhead at noon on June 21st (well, or the 22nd, depending on which day happens to be the summer solstice that year). The rest of the year it's slightly south of their zenith (overhead point) at noon. That's what defines the tropics, BTW -- the northern and southern extent of the tropics are the locations over which the sun will definitely be directly overhead at local noon sometime during the year. They go from 23.5 degrees S latitude to 23.5 degrees N latitude, because that's the tilt of the Earth, 23.5 degrees. More later or PM me. Good luck!

[watha]

This is great stuff. And the Unlikely Boat Builder is just fantastic. As entertaining as it is informative. Reading it helped me realize that the sun can't always be to the south of you in the northern hemisphere, because during summer, the earth's axial tilt causes us to "lean forward," and the sun would consequently be north of the equator, so portions of the northern hemisphere must be south of the sun's track. At least, I think that's right.

[clearsea]

Originally Posted by watha

the sun can't always be to the south of you in the northern hemisphere, because during summer, the earth's axial tilt causes us to "lean forward," and the sun would consequently be north of the equator, so portions of the northern hemisphere must be south of the sun's track. At least, I think that's right.

Correct. The sun is north of an observer on the equator when the sun's declination is north of the equator in the northern hemisphere from spring equinox to autumn equinox because of the inclination of the ecliptic (path of the sun through the sky) relative to the equator (23.4 degree tilt of the earth's axis relative to the sun).

This explains why the sun's declination (angular north/south distance from the equator) varies from 23.4 deg N to 23.4 deg S throughout the year, making the sun appear to move above and below the equator as the seasons progress. The sun moves northerly or southerly at about 1 knot during the two equinox periods (summer or winter), when declination is changing the fastest.

So, the sun is north of the observer (or sun-bather) in the northern hemisphere any time between the spring and autumn equinoxes when the observer is south of the sun's declination.

A navigator has to take declination into account when reducing a noon sun sextant sight to measure latitude.

Take a navigator on the equator (00deg 00.0'N) at longitude 064deg 0.00'W, on June 21, 2010 close to summer solstice, when the sun is as far north as it gets:

On that date and at that location, high noon was 13h17m48s DST. The sun was at an altitude of 66 degrees above the horizon, due north of the navigator. A person on land could have told us the sun was due north by looking at a sundial, but that device does not work well on a rolling boat, so the navigator at sea has to calculate the direction of the sun using sextant altitudes and tables of data. Or today's navigators can look at a computer with a GPS antenna attached to it to see where the sun lies relative to the boat.

On that near-solstice date at 1900 hours (7 PM) DST, the sun was about 4 degrees above the horizon and setting rapidly toward sunset at 1922, bearing 293.5 degrees from the navigator, or 23.5 degrees north of due west. Which makes sense, because the sun sets on the latitude of its declination north of the equator, and the declination on that date was almost 23.5 degrees north.

or ?

However, check this information with another source before relying on it. Hopefully someone else will step in to comment if I have mis-stated something.

[banyandah]

If I can be of help - Before SatNav or GPS, I sailed the world's oceans using just a compass and sextant for 16 years.

Sort of miss that connection with Earth and the heavens. Now I just look at my electronic gizmo, which I must add tells me exactly where I'm at day or night. Handy that.

I'm not sure whether this was covered in the above, but the Earth is tilted at 23 degrees 26minutes to its orbit around the sun, and therefore as we go around, at one point the northern hemisphere is tilted toward the sun and at the opposite point of our orbit, the southern hemisphere is titled towards the sun. Those points are called the northern solstice and southern solstice, and the sun would be pointing perpendicularly onto the Tropic of Cancer/ Tropic of Capricorn at those moments, which occur around the 21st of June and 21st of December. Above and below those imaginary lines, the sun will always be either north of an observer in the southern hemisphere, or south of an observer in the northern hemisphere.

Navigating by sextant is an art. The maths might be precise, being based on spherical triangles and the sun's perpendicular position on the Earth when the sight was taken, but the measuring of its relative angle to the observer when taken from the deck of a small boat, takes a bit of talent. Cook and Flinders were marvels at it. And I might add, so was I. I got paid to take specialist groups to mere specks of mid-ocean rock. Google Kingman Reef, a thousand miles from Hawaii. Not more than a dump truck load of clam shells poking out the ocean. There were plenty others. Mellish Reef, Barque Canada, Malpelo. But that was back in my first life. I still sail even though I'm pushing the years. And if interested, I'm about to launch my first book. So if I can help, I'm around most days.
Cheers From Banyandah