Planets Are Easy to Spot – When It’s Not Cloudy


The Sunday Nation


04 September 2005


After last week’s article dispelling the rumour that Mars will appear the same size as the moon, many readers have asked whether this planet can be seen with the unaided eye. The answer is yes. However, the weather is very cloudy therefore, this is not a good season for planet spotting.

Mars is one of the five so-called “naked eye” planets. The others are Mercury, Venus, Jupiter and Saturn. At the moment, Mars rises from the East at about 11pm and, by daybreak, it is in the overhead part of the sky. When the sun rises, the planet is outshone by the glare of daylight.

Saturn rises in the East at about 4:30am while Venus and Jupiter appear in the West immediately after sunset. Mercury is the most difficult to see; it rises only half an hour before sunrise and, before it goes very high, it is outshone by the morning sunlight.

Mars is easy to spot in the sky: it appears as a reddish, bright “star” that doesn’t twinkle like “ordinary” stars. Actually, all the planets don’t twinkle. To understand why this is so, we first need to know why stars sparkle.

Although stars are very large (a few million kilometres in diameter), they are also very far away. The nearest one is over 40 trillion km from the sun. As a result, their apparent size is so small that they appear as points of light.

When the light from these point sources reaches the Earth it first has to go through the atmosphere. Turbulence in the air high up in the atmosphere causes irregular bending of starlight.  As a result, the stars appear to be changing their positions and colours randomly. This haphazard movement is very small and it is what we perceive as the twinkle.

Although planets are small in size (a few thousand kilometres in diameter) they are also quite near (just a few tens of millions of kilometres). Therefore their apparent size is much larger than that of stars. The angular size of the “naked eye” planets varies from about 0.0015 degree (Mars and Mercury at their furthest points) to 0.015 degree (Jupiter at its nearest point).

To the human eye, the planets are seen as dots in the sky but not quite as points. The difference is that a dot has a small but distinguishable diameter while a point is completely dimensionless.

The light from the planets also undergoes the irregular bending as it passes through the upper atmosphere. However, since the source is a dot and not a point, the resulting apparent changes in position are not visible. Therefore, planets don’t twinkle.

This leaves one more question unanswered: The further away an object is, the smaller it becomes. At what distance does it appear as a dimensionless point? That is a story for another way

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