Leslie Merkel, age 11, of Kempton, Pennsylvania, for her question:

Do other planets have seasons?

All the sun's nine planets have seasonal variations, though in each case the pattern is different. The five outer planets are so bitterly cold that we earthlings would hardly notice the difference between summer and winter. On Venus, we would find both summer and winter unbearbly hot. Mars is perhaps the only other planet where we could appreciate the slightly warmer summers that follow the long cold winters.

Naturally the inner planets are hotter because they are closer to the sun. However, no planet has a perfectly circular orbit    so its distance from the sun varies through¬out the year. The elliptical orbits of Mars and Saturn, Uranus and most likely Pluto play a part in creating seasonal changes. But this does not account for all the differences between summer and winter. The tilt of a planet's axis is equally, and sometimes more, important in causing variations in a planet's seasons.

As planets go, the earth's orbit is fairly round. However, we are some three million miles farther from the sun in July than we are in December. If distance from the sun were the only factor, we could expect our midsummer season along with the Christmas trees. Actually this happens only in the Southern Hemisphere. Our alter¬nating seasons north and south of the equator are related to the tilt of the earth's axis    and this factor creates a much wider range in our seasonal changes. The axis around which the earth rotates is tilted at an angle of 23 1/2 degrees toward the plane of its orbital path around the sun.

Saturn, Neptune and especially Uranus are tilted even more sharply. For this reason, these planets have very marked seasons. The axis of Uranus is tilted so far that the big, fat planet rotates almost lying on its side. Its year is as long as 84 earth years and during this orbital period its distance from the sun varies by 166 million miles. Each of its four seasons lasts through about 21 earth years. Neptune's orbit is almost circular. But its sharply tilted axis causes four alternating seasons in each hemisphere. Jupiter's axis is only slightly tilted. But it has seasonal changes because its distance from the sun varies some 33 million miles. Saturn's axis is tilted more than the earth's and its distance from the sun varies by 95 million miles. Each of its four seasons endures through 7 1/2 earth years.

All the seasons on these outer planets would seem unbearably cold and unendurably long to a pampered earthling. But on Venus, the coldest winter night is not likely to dip below 600 degrees Fahrenheit. The orbit of the golden planet is almost circular. Its axis is tilted at less than half the angle of ours and its surface is shielded by a thick, dense atmosphere. Hence the seasonal changes on Venus are likely to be very slight. From our point of view, the most understandable seasons are on Dears. The red planet has a tilted axis and an oval orbit. Its scanty atmosphere makes it possible for telescopes to show its white polar caps, alternating with the seasons in the opposite hemispheres. However, the Martian seasons are much cooler and almost twice as long as the four seasons on our comfortable home planet.

The axis of Mars is sharply tilted and its distance from the sun varies by 25 million miles. These factors make the alternating seasons unequal. The longest season, lasting 199 days, brings fall to the Southern Hemisphere and spring to the Northern Hemisphere. The shortest season of 146 days brings the southern spring, and the northern fall. The south has more extreme winters because at this season Pears is farthest from the sun.