Wistow cum Newton Harcourt Parish Meeting

Serving the people of Wistow and Newton Harcourt

Clerk: Bal Bahia

Jupiter and more

The brightest 'star' in the southern sky at around 10pm is actually the planet Jupiter. Jupiter is surrounded by many moons; but 4, Io, Europa, Callisto and Ganymede can be seen with a small telescope or binoculars. If we have several successive clear nights you will be able to see that these moons change position as they orbit the planet.
Indeed such observations can be used to measure the orbital periods of the moons and thus predict their future positions and in particular their eclipses behind the planet. This is best done when Jupiter is opposite the Sun, and the Earth/Jupiter distance is not changing much. As the Earth orbits the Sun, away from this opposition point, the Earth/Jupiter distance will increase and the moons' eclipses will appear to arrive late due to the increased light travel time between Jupiter and the Earth. This can be as much as 16 minutes (twice the Earth Sun light distance) and was used by Roemur in 1676 to measure the velocity of light.
The Sun/Earth distance in, say, kilometres, can be measured by triangulation using 2 widely separated places on the Earth's surface. Hence the velocity of light can be measured.
The modern day value of the Earth/Sun distance is 150 million kilometres and the velocity of light is 300 thousand km/sec. Divide the former by the latter and you get 500 sec or 8 min 20 sec.
Strangely it may only take 8 min for light from the Sun to reach the Earth but it takes 100 million years for the energy generated at the centre of the Sun to reach the outer edge of the Sun. However, that's another story.


The Earth moves round the Sun in an elliptical orbit at a slightly non uniform rate. This causes a sundial to disagree with GMT by up to +/- 15 minutes.GMT is defined by an artificial sun called the Mean Sun, hence Greenwich Mean Time. The Mean Sun and the real Sun coincide at the Equinox which falls on the 20th March this year. Thus on that date a sundial on the Greenwich Meridian will read perfect GMT. Newton Harcourt is about one degree west of Greenwich, so a sundial will be 4 minutes behind GMT. Thus your sundial should read exactly 12 noon at 12h 04m GMT, similarly for any other hour. If it is forecast to be cloudy on Monday, try Sunday or Tuesday - it will not make a big difference.

Sundials do not do BST.