pyephem | Scientific-grade astronomy routines for Python | Dataset library

Check whether you have reversed your latitude and longitude. By swapping those numbers, you have asked about a different location on the Earth’s surface than you intended.

You are providing floating point numbers to radec_of(), and PyEphem interprets floating point numbers as radians, not degrees. Only when numbers are supplied as strings does it interpret them as degrees. So you could try either:

The Skyfield documentation describes getting the lunar longitude and latitude of the sub-Earth point here:

https://rhodesmill.org/skyfield/planetary.html#computing-lunar-libration

It looks like if instead of (earth - moon) you also did the same thing but with the difference (sun - moon), you would get the sub-solar point on the Moon. I’d suggest trying each of those out, and seeing if the values you get back match example values from some other authority you could check against to make sure you're getting values that mean the same thing.

(And, if that approach works, let me know by responding here with a comment, and I'll update the documentation to add a heading to that page of the docs that explicitly mentions the word “sub-Earth point” or “sub-solar point” — since I don’t think the word “libration” makes it obvious to folks needing sub-points that the section will answer their question.)

A rough back-of-the-envelope calculation would suggest that the Moon, circling the sky in 29 days, moves about 360° ÷ 29 ≈ 12½ degrees per day.

Your Skyfield script therefore has a problem: it only prints the date if the Moon’s longitude relative to the sun is between 6° and 14°, a range only 14 − 6 = 8° wide. In some months, its 12½° jump happens to land in that narrow 8° range, and some months it jumps entirely over it instead. That is why the Skyfield script only prints some months but not others.

The PyEphem script, by contrast, takes the angle in radians and multiplies by 30, turning the range 0…6.28 into the range 0…188.4. Very roughly, the unit of measure you are using is a “double degree” with 188 of them to the full circle. The Moon only jumps about 6¼ “double degrees” per day, so it is guaranteed to land in your 6-to-14 range at least once each month, because a jump of 6¼ is not enough to jump fully over the range.

My guess is that you should try an approach of choosing an exact degree angle that’s of interest to you, like 6°, and then write a little routine to find exactly the date and time it crosses that threshold each month. Here’s an example that should hopefully get you started:

https://rhodesmill.org/skyfield/searches.html

If you try printing out the PyEphem observer location:

Set your lat & lon as strings and format the alt & az as strings:

PyEphem does not, alas, have that capability.

From the front page of the docs:

PyEphem does not interoperate with NumPy and so is awkward to use in a modern IPython Notebook.

This basically means that the next_rising and next_setting methods can only operate on scalars. The quick and dirty solution is to write a loop to convert each element of your index to a compatible format and compute the values that way: