# Planning observation: determine dates where moon is within 30 degrees of my object?

I'm planning an observation for a proposal and I need to determine which dates the moon is within 30 degrees of an object so I can ask to not be scheduled for those dates. This object is near the ecliptic so it is guaranteed the moon will pass close by at times.

In the past I've used skycalc to manually figure out such dates, but now I need to figure out dates over a several month period, and I was hoping there was some more automated tool I could use for this purpose.

• How precise do you need to be? I would guess that "within 30 degrees" is just an estimate. Therefore, you can find one time interval and add 27.322 days to it (the mean sidereal period) to find future dates with reasonable precision. Sep 26, 2018 at 23:27
• 30 degrees is a selected but hard boundary. Sep 27, 2018 at 0:11

I would use JPL Horizons to calculate the position of the Moon and save the results to a text file. Then use a spreadsheet to calculate the separation between the Moon and your object. $$\cos \theta = \sin \delta_1 \sin \delta_2 + \cos \delta_1 \cos \delta_2 \cos (\alpha_1 - \alpha_2)$$ where $$\theta$$ is the separation, $$\delta$$ is the declination, and $$\alpha$$ is the right ascension.