Planets Are Easy to Spot – When It’s Not Cloudy
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
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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