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!

Citizen Science and the Occultation of Regulus by Erigone

Last night at about 2:12am EDT the asteroid Erigone blocked the light of the star Regulus for observers in parts of the northeast.  From the observer's point of view, Regulus would appear to wink out for a period of up to 14 seconds.  Such an event is called an occultation. 

On the face of it, seeing a star vanish briefly seems an unlikely thing to get up in the middle of the night for, but it is true Citizen Science.  With nothing more than a stopwatch* and an accurate clock, we could contribute to astronomer's knowledge of this asteroid's exact orbit, its size, and even whether or not it has a satellite--a "moon" of its own.   Enough observers in the right places would allow determination of the size of Erigone's shadow--which is the size of Erigone itself--while brief occultations outside of this belt of shadow would likely show the presence of a companion.  Even observers in New York City (which was in the path of the shadow) could contribute, since Regulus is bright enough to be seen even there.

And consider the idea itself.  Astronomers know the orbit of Erigone (a 45-mile-wide rock way out in the main asteroid belt between Mars and Jupiter) well enough to line it up with a hot, powerful young star so far away (at 79 light years) as to be a mere bright point of light, and locate the shadow they cast to a region of the earth stretching diagonally across New York State and bits of Connecticut and Pennsylvania in a time nailed down to a few minutes.  The first prediction of this occultation was made ten years ago.

This is "pocket billiards" of a very high order!

Leo, with Regulus the brighest star in the lower right.

 http://www.mediapolis.k12.ia.us/middle/middle/Todd%20H/6th%20grade/2008%20students/Jupiter%20AF/space/constellations/constellations.htm

(The disk-like star images are a telescope artifact--by eye, they're all bright but tiny points.)

Alas! --I missed it.  After reading up on the event, figuring out the stop watch function on my wrist watch, and setting my alarm, I looked outside to find solid overcast.  And by this morning it was raining.  Mind you, eastern Massachusetts is well outside the occultation zone of Erigone itself, but I cherished the idea of being part of the effort to see if she might possibly have a satellite.  Even if--as was very likely--I saw nothing at all but Regulus shining steadily out of the constellation Leo, that would be just as useful: it would tell astronomers one place where a satellite definitely wasn't.  Combined with the simultaneous observations of hundreds of others, it could have given astronomers a pretty clear idea of the Erigone "system."

Since this was to have been "the best and brightest asteroid occultation ever predicted to occur overa populated area," I am a little bummed.

*Even the stopwatch isn't strictly necessary: we are advised that simply counting aloud, "one-cigarette, two-cigarette,..." allows accuracy to about a quarter of a second.

Not "Wild" wild, but very pretty!

D. W. Field Park spans a large north-south area around Brockton's northern border.

I haven't walked in D.W. FieldPark in a long time--shocking, considering how close I live to the Jewel of the city.  Today is incredibly warm and sunny, and my youngest son and I took the dogs there as an excuse to visit.

The park is paved in loops with lanes for both driving and pedestrians.

A low hill makes a perfect vantage.

The park is arranged around a series of ponds.

Stephen has both dogs so I can take a photo.

Golda and Linkin had a ball, except that every other dog owner in the city seemed to be there.

(Seeing another dog, Linkin especially becomes a dogicidal maniac.)

Park benches make nice places for contemplating the landscape.

A "lantern tree" (American beech).

I've been putting down D.W. FieldPark as I've lately emphasized Wild Places, forgetting how beautiful it is.  Although a designed landscape (inspired by Frederick Law Olmsted) which may be heading for a makeover, I would now call it fairly "naturalized," though of course it is still managed--witness the obvious black top, and slightly subtler marks of the chainsaw.

On the other hand, I am considering managing my own little Wild Place come spring, tearing out as much of the invasive English ivy, vinca, and European buckthorn as I can, along with a few of the smaller Norway maples (also invasive); and planting a few native trees like white pine.  I will be trading a bit of land compromised in one way, with land compromised in another, more tolerable, way.  Less wild, more "wilderness."