Thursday, March 20, 2014

Happy Equinox! (12:57 EDT)

In what direction does the sun rise?  In what direction does it set?  I often run into those who are not sure whether the sun rises in the east or west!  But for the rest of us, this can be a bit of a trick question, since it depends on the season.  It is tied up also with the path the sun takes across the sky, and helps to explain seasons from an earthly point of view.

Let's unpack it by considering different situations.  In the simplest case, we are standing on the equator, and it is the beginning of spring.  The sun will rise directly east, pass directly over our heads at noon, and set directly west.  As the year lengthens towards summer, the sun rises and sets more and more northerly.  As summer turns toward fall, the sun returns to rising straight east and west, then after the beginning of fall the sun begins rising and setting south of east and west. 

We here in the temperate zone (outside of the tropics, which are bounded by lines running 23½ degrees* north and south of the equator) see a similar pattern even though we never see the sun directly overhead: it is low in the south in winter, of course, but even in summer never comes near to being overhead.  But the direction it rises and sets still depends on the season.  At the beginning of winter (the winter solstice, on or about December 21st) the sun rises south of east, climbs to low in the southern sky at noon, and sets south of west.  This is true everywhere.  At the beginning of summer (summer solstice, June 21st or so), the sun rises NORTH of east, crosses all the way to a point high in the southern sky at noon, and then sets NORTH of west.  In other words, the sun makes a much longer path across the sky on June 21 than on December 21, so that it is in the sky much longer.  Of course, we know that early summer days are the longest and early winter days the shortest, giving us more or less heat in that time period, but now you can see the reason in terms of the sun's path across the sky.  (In addition, that path looks down much more directly in summer, and those more direct rays heat the ground more effectively.) 

From our point of view, the sky is a dome!  Explaining solstices, equinoxes (and seasons) in one graphic.

Now to the original question: in what direction does the sun rise?  Since today is the vernal equinox--exactly between the solstices, at the point in earth's orbit when the equator is aimed directly at the sun, giving us equal night ("equi-nox") of twelve hours all over the globe--today all over the earth the sun rose east and will set west.**

A final note: with the equinox occurring  almost exactly at local noon means that we actually "saw" the sun cross the "celestial equator" and move into the north "celestial hemisphere."  And today is divided almost exactly into a winter morning and a spring afternoon.  

A pity its overcast!

Here is the situation in space.  Imagine the universe projected on a sphere that surrounds the earth (the celestial sphere).  The ecliptic is the plane of our orbit around the sun.  The celestial equator is a projection of the equator on the celestial sphere.   Remember that the earth's axis always points in the same direction (coincidentally, at the north star).  In the model above, the earth spins, and the sun sloowwwly makes its way along the yellow line.

"Star trails" made on a long-exposure photograph.  The star near the center is the north star,
aka Polaris; that it does not appear to move shows that Polaris is in line with the earth's axis of rotation.
(No, you're right: I don't suppose the photo was taken here in Massachusetts.)

*which is the earth's tilt on its axis relative to the plane of its orbit around the sun.

**Okay, really the equinox is a point in time, not a whole day, but this is east and west near enough not to matter!

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