Astrological ephemeris for Charon

Instructions below. Read them first.

Instructions

This page calculates the position of Charon as viewed from Pluto with a light-speed correction for Earth, in the way that I think makes most astrological sense; see my page on Charon astrology for more information. If you want the position of Charon as viewed from Earth, just use any standard calculation of Pluto's position - Charon is so close to Pluto as to make no difference. Please note that Charon moves fast, with an orbital period of 6.4 days; so correct time and time zone are important. That is also why a Charon ephemeris presented as a book or table wouldn't work out too well.

Comments and suggestions welcome. Times and dates entered on this page are not stored. Although I have made every effort to make sure this page produces correct data, it is hard to verify and may be partly or completely wrong; do not use it for spacecraft guidance or similar purposes! In jurisdictions that require such a statement, astrological information is provided for entertainment purposes only.

My data source is NASA's PLU013 and DE414 ephemerides, which only give Charon data for the years 1965 to 2050. This page calculates positions for 1900 to 2100, using an analytic model fit to the PLU013 data. The fit looks very good and I'm confident that it'll be pretty accurate even outside the 1965-2050 range, but be aware that results outside that range are somewhat speculative. There just aren't enough observations to allow a really precise calculation - NASA's numbers aren't necessarily that much better than mine, and it didn't inspire my confidence that while creating this page, I discovered and reported, and they acknowledged, a bug in their ephemeris calculation software. Some accuracy is lost (primarily in the short-term range) due to the approximations necessary to create this Web page, too. I don't have the kind of hosting where I can put random FORTRAN programs and megabytes of lookup tables behind a Web page. See details of what's included, below.

This calculation includes:

• Civil time to UTC (exact by definition)
• UTC to dynamical time (matches observations for the past, and Swiss Ephemeris prediction for the future, to 10 seconds of time over 1900-2100)
• Earth-Pluto light time (matches DE414-PLU013 composite to 30 seconds of time over 1900-2100)
• Two-body model fit to Charon-Pluto system (matches PLU013 to 7km over 1965-2050, which is all available data)
• J2000 equatorial to ecliptic coordinate transformation (constant value; should be good to a tiny fraction of a second of arc)
• Precession of Earth's mean equinox (should match any standard reference to better than one minute of arc)

It does not include:

• Higher-order Earth motion effects (nutation, etc.)
• Location on the Earth's surface (these are geocentric coordinates)
• Any atmospheric effects
• Any relativistic effects
• Any form of "aberration"
• Pluto-Charon light time

The target accuracy is plus or minus 30 minutes of arc, though in many cases the result may be better.