This won’t happen again in our lifetimes apparently. Venus, as I write is transiting the sun and a few moments ago I got to see it. It’s a very small dot on the sun but viewable none the less unmagnified. But you HAVE TO wear eclipse glasses or welding glasses to protect your eye and in any case the glare is too high if you try with out. Just so happens that next week we are sailing on our Bareboat Charter Master post graduation sailing trip to Tahiti. This place has historical significance with Venus transits. Captain James Cook was commissioned by the Royal Astrological Society to sail to the south pacific (Tahiti) to observe the transit in 1769. The mission was to note the time that the planet entered and exited the sun’s circle. This was also performed by others in other various places on Earth and the attempt was made to use some pretty amazing for it’s time geometry to help calculate the distance to Venus and to the Sun. A few months later, Mercury also transited the sun. Cook happened to be in New Zealand at that time and also made that observation. I plan on visiting the bay where Cook made the Tahitian observation next week.
Transit of Venus Across Sun June 6th 2012
This snippet from the Guadian.co.uk website.
[In 1769] Why was Halley so eager that hordes of astronomers disperse to the ends of the Earth decades after his death to watch a planet pass between the Earth and the sun?
Halley realized that accurate measurements of the time it took Venus to cross the sun, when taken from distant points on Earth, could be used to establish the distance between the Earth and Venus, and thus the distance between the Earth and the sun.
The 17th century German mathematician Johannes Kepler had already calculated the proportional distances of the solar system. Thanks to Kepler, scientists knew that Jupiter is five times as far away from the sun as Earth is, for example.
They knew the proportional distances. They just didn’t know the absolute distances. This was the central quest of astronomy: to determine the size of the solar system.
And suddenly Halley saw how it could be done, using the transit of Venus.
If you’re interested in Celestial Navigation, take the NauticEd Celestial Navigation online course.
The posting here is not a course in celestial navigation by any means. However it’s meant to simplify a few principles for you so that you’ll at least have some sort of celestial orientation. And… perhaps it’ll inspire you to learn the aging art.
This was written by Grant Headifen, Educational Director of NauticEd. NauticEd provides online sailing courses and Sailing Certifications accepted by charter companies worldwide.
Latitude: In the northern hemisphere, finding latitude is simple using one of the greatest gifts to human kind – The North Star. What ever angle the northern star is at from the horizon, that’s your latitude.
Imagine you’re an ant sitting on the top of an apple looking at a spot directly above you on the ceiling then the spot is 90 degrees from the surface you’re standing on. If you’re standing half way around the apple then you’d barely see the spot but it would be horizontal to the surface you’re standing on and so the spot would be at zero degrees. And if you were ¼ of the way down the apple then the spot would be at 45 degrees etc. ie the northern star is the spot on the ceiling to us.
You can also find latitude using other celestial sightings but they involve table lookups and are slightly more complicated. Not meant for this post and also note that there are a few more complicated variables not taken into account during this simplistic explanation like the height of your eyeballs above the earths surface etc etc. But at least you’ve now got the principle.
Longitude: Now this is a fun one and in an incredibly easy principle. But years ago (early 1700’s) while the principle was easy then the execution was difficult. Read on to see why.
The earth rotates through 360 degrees in 24 hours. That’s 15 degrees per hour. By convention, when the sun is at it’s highest point in Greenwich, it is noon in Greenwich. That means that at a place that is 15 degrees to the West of Greenwich the sun will be at it’s highest point one hour later. Six hours after Greenwich the sun will be at it’s highest point somewhere in over the USA and 12 hours later the sun will be at it’s highest point in New Zealand.
Animation of time zones
So if we know the time in Greenwich and sun just reached its highest point where we are then we can calculate our longitude.
Lets do a few examples. If it is 6 pm in Greenwich and the sun just peaked overhead here, then I am 6 x15 degrees to the west of Greenwich which is 90 degrees West which is right near St Louis Mo.
If the sun peaked overhead in Los Angeles what time would it be in London.?Well LA is 118.15 degrees West (from Google earth). Divide that by 15 degrees per hour and we get 7 hrs 53 minutes. Now since the times zones are created in bands this would round up to 8 hours. Thus it would be 8pm in London.
You’re sailing in the Greek islands in the Mediterranean and a little bird just told you your latitude is 34 deg 54 minutes north but failed to tell you the longitude. Fortunately you have your handy sextant and just as you take a shot, the sun just reached its apex overhead. You look at your watch and the local time is 12:10:48 pm. Where are you?
Since you’re in time zone B you are 2 hours ahead of Greenwich. Thus the time in Greenwich is 10:10:48 am. And since the sun peaked just now (=noon) then you are 12:00:00 minus 10:10:48 = 1 hour 49 minutes and 12 seconds from Greenwich. Putting this into decimal time this is 1.82 hours. Multiply this by 15 degrees per hour and we have 27.3 degrees East or 27 degrees, 18 minutes East.
You’re in the harbor north of the town of Kos on the Island of Kos.
That was incredibly easy, so why all the hoopla back in the 1700’s? The King of England even offered up a ₤10,000 reward to anyone who could solve the issue of Longitude. The above math was well known but the issue was telling the time. No one could accurately keep time at sea. After 27 years of work on the project, John Harrison, finally invented the Chronometer more commonly known as the watch. The watch was not susceptible to the sudden crashes of waves at sea and thus kept proper time.
James Cook on his second trip around the world in 1772 sailing on Rendezvous, took Harrison’s watch with initially much skepticism. Stating that he’d give it a try. After six months at sea, Cook stated that the Chronometer would almost certainly become the way of the future for Navigators. Cook then went on to reposition many of the Islands in the Pacific including Tahiti, his favorite island. His map of New Zealand astounds people even today with its accuracy.
Again there were a few simplistic assumptions taken in that explanation. But now, at least you understand the principle of longitude determination from a noon shot of the sun. You can also determine your latitude from a noon shot of the sun as well using tables and a bit of math. Again beyond this posting.
If you’d like to delve deeper into these topics, NauticEd provides online sailing lessons and an Introductory Celestial Navigation Sailing Course, or maybe you’re just happy with your handy boring ol GPS.
Well here it is – a much requested clinic on Celestial Navigation. The Introductory clinic gives a good introduction to spherical geometry and the theory of a noon solar sight and takes the student through an actual sighting using real tables to calculate the Latitude and Longitude of a vessel at sea.
The clinic is quite simple to understand and most anyone with a slight yearning to understand how sailors of old were able to do it – will beable to get through the clinic.
Please enjoy the NauticEd Introductory Celestial Navigation Clinic – authored by Captain Ed Mapes