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Campden Skies, July 2007

Early this month in the twilight just after sunset look for Venus still bright in the west. Then look a little more and not far away you will find another object shining without twinkling. That will be the planet Saturn. Both will have disappeared into the sunset glow by the end of the month.

Planets appear to move amongst the fixed stars and so attracted the attention of the early civilisations. Saturn was named by the Romans for the father of the king of the gods, Jupiter. This planet was the most distant known to the ancients and travelled through the stars more slowly than any other planet they watched. Thus it seemed logical to name it for an elderly and presumably slow-moving god. The planet speeding fastest was called Mercury - the messenger of the gods whose sandals were winged. Venus is the brightest and most beautiful planet visible from Earth and not surprisingly was named for the goddess of beauty and love.

As you will see, Venus is far more brilliant than Saturn. Indeed you may not be able to make out the latter until the twilight begins to darken. Does this mean that Venus is bigger than Saturn? By no means - it is just an optical effect of distance. Just as a candle in your hand would seem brighter than a searchlight miles away so nearby but tiny Venus outshines gigantic but distant Saturn. Both shine by reflected sunlight.

Venus orbits the Sun inside the Earth's orbit and so shows phases, like the Moon. At present it is crescent shaped and you may be able to see this with binoculars - after sunset. As the month progresses the planet will not appear any dimmer even though the illuminated portion will shrink day by day as it gets ever closer to the Sun. This is because it is coming closer to Earth and the disc is getting larger. For all its brilliance it is a little smaller than the Earth, another rocky planet and with complete cloud cover that reflects much of the sunlight. Its diameter is 12,104 km. (The equatorial diameter of the Earth is 12,756 km and the polar is 12,714 km.)

In between Earth and Saturn lie the orbits of Mars, the asteroids and Jupiter. It is a gas-giant very far away. At present it is on the opposite side of the Sun to us so its apparent closeness to Venus is just due to the effect of the line-of-sight. Its equatorial diameter is 120,000 km and the polar is 107,100 km. These figures do not include its rings which can only be seen though a telescope. In spite of its size, Saturn's mass is not as much as might be expected. If an ocean large enough could be provided, Saturn would float! It is far enough from the Sun to travel at only a little under 10 km a second.

The strength of the pull of gravity falls off with distance and that means the closer to the Sun the faster an object must orbit to avoid falling in. Earth orbits at just under 30 km a second but Venus has to keep up fractionally over 35 km a second. Mercury travels at just under 48 km a second. No wonder that another name for mercury is quicksilver.

If you are a bit giddy at the thought of the speed you are travelling, add in the effect of the daily rotation that whizzes you round many thousands of miles every 24 hours and the motion of the Sun itself, dragging its collection of planets, orbiting the centre of the Galaxy; and we must not forget that the Galaxy itself is moving within its cluster of galaxies. That cluster of galaxies is no doubt heading somewhere but we'll leave it there as I'm a little dizzy.

Jill Wilson
(From the Chipping Campden Bulletin. Included with kind permission of Jeremy Green)

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