Computing sub-solar point

问题

I am just getting started with PyEphem. My immediate task is, given a date and time compute the sub-solar point on Earth with latitude-longitude values. I'll dig into PyEphem to work this out but if someone has already done this, I'd appreciate sample code.

回答1:

I cannot test actual code from where I am this morning, but: an object at declination ϕ should always ride right above the series of locations on earth that have latitude ϕ, so the latitude number is given to you directly by a body's .dec attribute (or .a_dec or .g_dec depending on your application).

Now, what about longitude?

Imagine the situation, which I suppose must occur roughly once a day, when Greenwich at 0° longitude looks up and sees the line in the sky of 0° right ascension right overhead. At that moment, a body in the sky at right ascension θ would be looking down at longitude θ assuming that longitude is positive going east, as is the case with PyEphem.

Now, what if Greenwich is looking up at a non-zero line of right ascension instead? Then it seems to me that we just need to subtract that from a body's right ascension in order to make longitude, because as the day proceeds and the Earth turns and lines of right ascension pass over Greenwich with bigger and bigger right ascensions assigned to them, any given body is going to pass west across the Earth and its longitude will dwindle and then go negative as it passes over the Western Hemisphere.

The line of right ascension overhead at Greenwich at any given moment can be determined by creating an Observer at 0° longitude and asking for its .sidereal_time() if I recall the Quick Reference correctly. So I think that the longitude beneath of a body might be:

lon = body.ra - greenwich.sidereral_time()

I will do a quick test with this later on today to see if reasonable numbers come out.

回答2:

I went looking for the same answer as the OP. Many posts "mention" how PyEphem is the way to go but without providing the actual example.

Here is my working example to calculate the subsolar point. Mapping everything to a longitude between -180 and + 180 degrees.

greenwich = ephem.Observer()
greenwich.lat = "0"
greenwich.lon = "0"
greenwich.date = datetime.utcnow()
sun = ephem.Sun(greenwich)
sun.compute(greenwich.date)
sun_lon = math.degrees(sun.ra - greenwich.sidereal_time() )
if sun_lon < -180.0 :
  sun_lon = 360.0 + sun_lon 
elif sun_lon > 180.0 :
  sun_lon = sun_lon - 360.0
sun_lat = math.degrees(sun.dec)
print "Subsolar Point Sun Lon:",sun_lon, "Lat:",sun_lat

I am no expert in PyEphem and there may be a better approach - but my testing so far has this work for my purposes.

p.s. yes.. Greenwich above is not actually set to the actual lat/lon... it's really only the Longitude of 0.0 that's needed to get the appropriate Sidereal time we need.

来源：https://stackoverflow.com/questions/17262428/computing-sub-solar-point

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