The Sun
Peter will be walking in England and Scotland and will often need to rely on the sun to maintain a sense of direction. You will remember the British Isles is situated in the Earth’s Northern Hemisphere and as the season changes from Spring to Summer and then finally at the end of the walk to Autumn the sun will gradually increase its height in the sky at midday and then fall. It will rise each day as he walks east and north until reaching its zenith on midsummer day on about 21st June and then fall back as he walks west and south.
Seafarers for hundreds of years used this aspect to determine there latitude. This was done by taking a sighting of the sun at midday and noting the angle of the sun above the horizon. The longitude was much more difficult to determine accurately as you needed to know the time at Greenwich in London from where an imaginary line of Zero was drawn round the world through the north and south poles. You needed to have a very accurate clock capable of running smoothly despite the storms the ship may encounter, so you could determine the time difference from the solar midday, that is what appeared to be 12 0’clock where you were and the time per your clock. Noting the difference in hours and minutes you could work out your longitude. Thus if you thought it was midday, but your clock said it was already 13:00 (1:00pm) then you were 15° west of Greenwich. You would then sight the angle of the sun to get your latitude. However, as this angle changes daily you would need go to a specially created table and from the date determine what your latitude actually was.
This walk is purposely timed to arrive at the most northerly point when the sun reaches its highest point in the sky. However, as you will appreciate as I will not be standing on the Greenwich Meridian of Zero degrees the actual time will be about 16 minutes past 12:00 noon (or 1:16pm British Summer Time)
I have looked up what the angle of the sun will be when I reach each of the four compass points. It is interesting to note that as I travel east, north and then west the angle is not greatly different at E 53.10° N 54.77° W 55.69° but by the time I arrive at the southerly Lizard Point it has dropped to S 46.95°. The matter is further complicated by the fact that the earth does not spin round an exact number of days before returning to its same position in it’s orbit round the sun. It is about 365 and one quarter days, with 2012 being a ‘leap year’ we try to correct this by having an extra day on the 29th February. In practice the sun reaches its zenith on June 20th at 23:09pm; which indicates if you could see the sun by standing somewhere north of the Arctic circle it would continue its rise in the sky till nearly midnight.
The reason is quite difficult to understand but it is in part due to the earth spinning on a axis which is itself about 23° back from a vertical plain against the plain of motion round the sun. It is this aspect which gives us our seasons, with longer sunlight hours in the summer and shorter in winter. As you go further north in the summer you will get longer days while at the same time shorter days in the southern hemisphere. The change in day and night length speed up and slows down depending on where we are in our orbit round the sun; not because the speed of rotation changes but because of the parallel beam of light hitting the tilted spinning earth. You will notice after midsummer day which is usually 21st June the day length in England or Scotland will only shorten during the rest of June and the first part of July by 2 or 3 minutes but shortens more quickly as we go into August and by the time we get to the Equinox on September 21st the day is shortening by about 15 minutes. It speed of change will then slow down again as we move into November and December.
Some of you may think I have made a mistake as the sun will be slightly higher in the sky when I get to the most westerly point than it was at Dunnet Head in the north. However, remember the earth is tilted back so while it appears to get higher in the sky until midsummers day you can also “walk towards the sun” that is if I travel south as we move into summer its angle increases even more because it will actually be directly overhead at the Tropic of Cancer being at the latitude of a little more than 23° North from the Equator. The most westerly point is south of Dunnet Head by about 2° of latitude and as this will be only about 17 days later in early July the sun has yet to increase its speed of apparent descent back towards the Equator and onwards to the Topic of Capricorn. I suggest Professor Brian Cox or perhaps Sir Patrick Moore may be able to explain this better, particularly as they will be able to produce computer generated images or even actual pictures from space showing some of the aspects described above.