Show weekly ouide guide to science by permission ot of POPULAR SCIENCE MONTHLY KY DOES THE MOON SEEM TO FOLLOW YOU ir YOU WATCH IT WHILE tiou IOU ARE WALKING because it stays the same sa height height la in tile the sky any th thing in on C earth aran ilk like the top ol 01 f a church steeple seems to sink rink lower toward the horizon as you go 90 away from it ot of course the moon Is much too lar far away to do this it is as though it were on top of 0 a tower miles high that being the distance rom from the surface of the earth to the moon WHAT ARE THE WHICH MAKE WHAT WU WE CALL THE MAN IN THE MOON they are ara mountain ranges and plains some ot of the plains are arc a little darker colored than others perhaps because the rock on their surface Is a little different and this also makes some ot of the markings many ot of the moon mountains are great rings sixty or seventy miles in diameter 73 r and which astronomers believe f to be the craters ot of extinct CAN WE SEE ALL or WE THE MOON no we have never seen but one side ot of it the rear side is never turned toward us all that we know anything about Is the part ot of the moon that faces our way this visible part comeri comprises a burfe ce about six tenths ot of the moons surface instead ot of an exact halt half the moons axis tips a little from time to time and discloses a little more than one hemisphere HOW LARGE IS THE MOON IN comparison PAI tISON WITH TIIE THE EARTH the diameter of the moon Is 2160 miles which Is a little more than a quarter ot of the diameter of the earth this would make the volume of th the moon about one sixtieth that ot of th the sane earth but as s the substance of the moon Is a little lighter in proportion than the substance ot of the earth the moons weight Is only about one eight lath the weight ol of the earth I 1 WHY IVRY HO DO WE BELIEVE THE MOON IS A ERY COLD because it has bas no air during the moons day when the sun Is shining on its surface the rocks may be heated up a good deal but at night with no sun there Is nothing to keep out the cold ot of space and the surface of the dark moon Is probably or degrees below zero HOW DO WE KNOW THAT MEN DO NOT LIVE E ON THE THC MOON because it has no air and no water at least in liquid lorm form and because it Is so cold mil IT ARE COMETS they are clouds of gas perhaps with some solid fragments which come into the solar system irom from outer space most ot of them go out again and are never seen any more but P few have regular orbits around the sun run and come back again and again WHY DO THE COMETS SHINE most ol of their light Is reflected sunlight like the light ot of the moon but some ol of them have a taint faint light ot of their own these glowing ones usually contain hydrocarbon gases something like ordinary illuminating gas gag WHY DOES THE TAIL OF A coa COMET USUALLY POINT awa AWAY FROM TIIE biln buln because the tan tail la Is formed by the pressure ot of sunlight when light shines on anything it exerts a very slight pressure on that thing thine this pressure Is tar far too slight to be felt or measured in any ordinary way but it Is strong enough to push very tiny particles ot of matter away from the su n the taus tails of comets are made up oil such very tiny particles pushed back by the sunlight one recent comet showed however n R tall toward the sun nobody knows why |