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» Online Classroom   »   » Public Discussion of Cel Nav   » Venus transit of the sun

   
Author Topic: Venus transit of the sun
David Burch


 - posted June 01, 2004 11:03 AM      Profile for David Burch           Edit/Delete Post 
next week there is a venus transit of the sun. My guess is we can find longitude from this observation. Anyone interested, please do google search on "Venus transit of the sun" to find plenty of data and info.

then we can discuss here how to find longitude this way.... obviously not a method we can use very often! but the exercise should illustrate how to do this with other simimlar phenomena that do happen more often... moons of jupiter, star occultations, etc.

From: Starpath, Seattle, WA
David Burch


 - posted June 09, 2004 11:13 PM      Profile for David Burch           Edit/Delete Post 
We could not see it from Seattle (see article and visibile regions at Venus transit), but it would have been very sharp in any sextant telescope. Below is a blow up of it just beginning. I think it lasted about 6 hours.

 -

By noting the time on a hack watch when it first appeared, or estimating when it was halfway across, or when it left the other side, or by averaging all of the above, we could then compute from the almanac when it actually happened in GMT and thus set the hack watch to GMT.

Here is quote from an online news release.

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"Until 1:19 a.m. EDT, June 8, 2004, no living person had viewed the rare phenomenon first seen by British astronomer Jeremiah Horrocks in 1639.

From Sydney to the pyramids and western Europe, people armed for the occasion with pinhole cameras and special dark glasses gathered for the celestial show.

Venus appeared as a small black dot on the lower edge of the Sun at the start of a transit that ended about six hours later.

"We are watching the first transit of Venus since 1882," said Dr Robert Massey of Britain's Royal Observatory in Greenwich as more than 100 people thronged the courtyard of the London landmark to witness the phenomenon."

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... so we will have to wait till 2012 to try again!

the subject is still open for discussion if any one might be interested.

From: Starpath, Seattle, WA
Wendel Brunner


 - posted June 14, 2004 10:57 AM      Profile for Wendel Brunner           Edit/Delete Post 
Finding Longitude from the Transit of Venus

OK, I’ll bite on this.

You can certainly determine longitude from observations of the transit of Venus. To find longitude, you need to know the time. The geocentric times of the four contacts of the Venus transit are listed on the web site of the European Southern Observatory (www.eso.org), as well as other places. Observing the first through fourth contacts during the transit (or really any one of them) can yield the time, and in conjunction with an ordinary celestial sextant sight of a body near the east or west, the longitude.

Of course there are a few complications. The geocentric times listed refer to when the contacts would be seen by an observer located at the center of the earth, an inconvenient vantage point for astronomical observations. For observations from the surface of the planet, the times are affected by parallax of the sun and Venus. For ordinary celestial navigation, parallax of the sun and even Venus is not very important; the latter is a minor correction and the former is almost always ignored. For a Transit of Venus, however, parallax is a big deal. The historic reason to observe a transit of Venus was to determine the parallax of Venus. From that comes the length of the Astronomical Unit (AU), and hence, the size of the solar system, the baseline for stellar parallax, the distance to the stars, then to the galaxies, and, eventually, the extent of the Universe in which we find ourselves. So parallax here is important.

The same web site provides a table of coefficients and discusses an approximate formula from which the times of the transit contacts can be calculated for any position on the earth to within about six seconds. For example, first contact in Athens (37 58 N, 23 43 E) is at 5h20m3s UT, first contact in Catania, Italy (37 30 N, 15 5 E) is 17 seconds later. This table and formula suggests the following iterative approach to finding longitude from a Venus transit, similar to the one used to find longitude from Lunar Distance:

1) Set up your telescopes. If you are at sea, it would be best to put in at the nearest island and set up the telescopes on land. That is in essence what Captain Cook did when he observed the Transit of Venus on his first voyage in 1769. Otherwise, you could try mounting your telescopes in gimbals on the ship, as Galileo recommended. However, even Patrick O’Brian’s famous Jack Aubury was unsuccessful with a gimbal-mounted telescope when he tried it in from the deck of a ship of the line.

2) Determine the latitude by meridian passage of the sun.

3) Assume a longitude (AL) based on your best estimate of where you think you are. From that assumed longitude and a sextant observation of a body near the east or west, calculate the corresponding assumed time (AT). Or counter wise, assume a time and calculate the corresponding assumed longitude. Set your local watch to the assumed time. From the latitude and assumed longitude, calculate the times for the transit contacts.

4) Time the four contacts using the local watch. Try to time these contacts to the nearest second. Set up as many telescopes as possible to get independent observations, so the results can be averaged.

5) The difference between the observed and predicted times for the contacts serves as a correction for the local watch time. From the corrected local watch time, determine the corrected longitude. These corrections give a new assumed time and new assumed longitude.

6) Recalculate the predicted times of the transit contacts from the new assumed longitude. Compare to the new local watch time. Go back to step 5. I think one iteration will be enough, if the initial assumed longitude isn’t way off. At this point, you have Universal Time on your local watch to within 5 or 10 seconds, and longitude to within 1 –3 minutes of arc.

More accurate calculations of the transit contact times using the exact formulas could give a more precise determination of time and longitude. However, doing that is much more complicated. I suggest you use the the approximate longitude value to find the way to sail back home, and bring the observational data back for the Astronomer Royal to further refine. Picking up spare batteries for your GPS, or at least for your Casio watch, would also be a good idea.

Since a transit of Venus occurs less that twice a century, I think it better to rely on lunar distance to find longitude when the chronometer quits. Cook carried no chronometer at all on his first voyage; all his longitudes were determined by careful lunar observation. Measuring the distance between the sun and moon during the day, or between the moon and a star near the ecliptic at night, has been a historically practical way to find longitude without a chronometer, and is still a method celestial enthusiasts can emulate today.

Wendel Brunner
Berkeley, Ca



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