SkyTalk September | So Far Yet To Go

Two basic astronomy questions were recently asked in a survey taken of the graduating class of a prestigious New England university.  One was: “What causes the seasons?”  The other: “What causes the phases of the Moon?” Astonishingly, 86% did not know the answer to either one!

Although introductory astronomy courses are offered at most colleges and also many high schools as an elective, they are among the least signed up for.  “Too much math” is one of the excuses often given by students.  This misconception dates back a century or more ago when astronomy concentrated on the orbits of the planets and comets, and the positions of the stars.  This was before the birth of such fields as astrophysics, cosmology, astrobiology, and spacecraft missions.  The exciting discoveries in these areas overshadowed what little basic mathematics is required to understand them.

So, what does cause the seasons?  Most people know that as the Earth orbits the Sun it is sometimes closer and sometimes further from it due to our elliptical orbit.  And most assume it must be hotter in the summer because we’re closer to our Daytime Star and colder in the winter since we are further away.  Wrong—it’s just the opposite!  It’s the 23.5 degree tilt of the Earth on its axis that causes the seasons.  Here in the Northern Hemisphere, we are tilted more directly towards the Sun as we orbit it, causing its rays to be more concentrated (and therefore hotter) over a given area.  In the winter, we are tilted away from the Sun and its rays are more spread out (and less hot).  This situation is reversed in the Southern Hemisphere, which is why they are having winter when we are having summer.

  1. So now what causes the phases of the Moon? Basically, as it orbits the Earth it presents varying parts of its illuminated surface to us. When the Moon is “new” and not visible, it is between us and the Sun.  When it’s “full” it’s opposite the Sun in the sky (rising as the Sun sets) and is fully illuminated.  In between either of these extremes, it appears first as a crescent, then half-full, and finally three-quarters illuminated—then the sequence reverses after full Moon.  (BTW—many people are understandably confused by the designation “first-quarter” on the calendar when the Moon actually appears half-full in the sky.  That term comes from the fact that at first-quarter, our satellite is a quarter of the way around the Earth in its orbit.  At last-quarter, it’s three-quarters of the way along its monthly circuit.)

The above two examples show just how far we have yet to come in increasing public awareness of even the most basic facts about the planet on which we live and the sky above us.  To my mind, astronomy should be part of all school curriculums at every level of education!

— James Mullaney

Former assistant editor at Sky & Telescope magazine & author of nine books on

stargazing. His latest, Celebrating the Universe!, is available from