# Julian Ephemeris Date from Ephemeris Time

1. Apr 3, 2015

### Philosophaie

Julian Ephemeris Date (Teph) is equal to JDj2000 + Ephemeris Time (ET)(in seconds) divided by Seconds per day.

Teph = j2000 + ET / spd

How do you find Ephemeris Time?

deltat = ET-UT = 24.349s + 72.3165s * T 29.949s * T + 1.821 * B ...

http://www.digplanet.com/wiki/Ephemeris_time [Broken]

How far off is Julian Date from Julian Ephemeris Date?

Last edited by a moderator: May 7, 2017
2. Apr 3, 2015

### marcus

I'm not sure what you want to convert to what. Could you boil it down to one clear problem?

There is a computer program DaVinci that converts between UTC (universal time) and ET (ephemeris time)
http://davinci.asu.edu/index.php?title=ET_UTC

Here are online calculators which convert from CALENDAR DATE+Universal Time (UTC) to Julian Date and vice versa
http://aa.usno.navy.mil/data/docs/JulianDate.php
http://www.onlineconversion.com/julian_date.htm

I'm not an expert and can only tell you my impression about Ephemeris time: there seem to be several versions of ET and the main one AFAICS is centered at the barycenter of the Solar System which means that TERRESTRIAL time counts are going to be slightly different because of RELATIVISTIC CORRECTIONS having to do with the earth's motion.

Since the earth is moving relative to the sun, a standard time constructed for the solar system, space navigation, orbit modeling etc etc, like ET (developed by JPL) is going to be just slightly different from time measured by clocks on the earth's surface which are moving at variable speeds around the sun.
Even an idealized clock at the center of the earth, which didn't have corrections due to rotation, would still be different from "standard solar system time"

But these relativistic corrections are so tiny I would not worry about them unless you have some good reason to care.

Just my two cents.

Last edited: Apr 3, 2015