Page 128 - SST Class 06

Rotation and Revolution of Planets
Planets Mean Distance from the Sun Period of Revolution Period of Rotation
in Millions of kilometers

Mercury 57.9 88 days 59 days
Venus 108.2 224.7 days 243 days
Earth 149.6 365.2 days 23 hr, 56 min, 4 sec
Mars 227.9 365.2 days 24 hr, 37 min
Jupiter 778.3 11.86 years 9 hr, 55 min, 30 sec
Saturn 1,427 29.46 years 10 hr, 40 min, 24 sec
Uranus 2,870 84 years 16.8 hours
Neptune 4,497 165 years 16 hr, 11 min

Varying Lengths of the Day and Night : We depending on which hemisphere is inclined
have already seen how the days and nights are towards the poles depending on which hemi-
caused in the course of the rotation of the earth. sphere is inclined towards the sun at the time. At
The circle of illumination does not exactly cover the poles, it can be either six months continuous
half of the earth. The light is always spread over daylight or six months continuous darkness. This
more than half of the earth. Less than half of the means that days and nights are of six month’s
earth is always in darkness. While the point of duration at the poles. This happens with such
darkness and sunlight is reached every 24 hours, precision that duration of days and nights from
sun light will be longer than the period of time equator to pole and from one part of the year to
under darkness. Thus, when the Northern another part can be indicated in terms of days
Hemisphere remains inclined towards the sun, and months of the year.
the days will be longer and nights shorter. At the March 21 to September 23 : During this
same time, opposite happens in the Southern period, the Northern Hemisphere remains
Hemisphere where the nights will be longer and inclined towards the sun. The North Pole receives
the days shorter. When the Southern Hemisphere sun light continuously for six months. During this
is inclined towards the sun, all places in the period, the Southern Hemisphere, moving
Southern Hemisphere will have longer days and onward from equator, hours of daylight go on
Northern Hemisphere or the Southern increasing. It is 15 hours daylight at 41° latitude
Hemisphere remains inclined towards the sun, and 20 hours at 63°4' latitude and 24 hours
the Equatorial portion remains always focused daylight at 66°5' latitude and goes on further
towards the sun in a manner that the effect of increasing to 6 month’s duration at 90° latitude
inclination is hardly felt. The effect of inclination (The North Pole).
goes on increasing as we move towards the On two day – March 21 and September 23 – If
poles. neither of the poles is inclined towards the sun or

Because of the above phenomena, days and away from it. On these two dates, days and
nights are always of equal length, —of 12 hours nights are of equal duration not only on the
duration each—at the equator. The length of the equator, but throughout the world. These two
day goes on increasing towards the poles days are known as Equinoxes.

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