# Earth-Moon Barycenter Perihelion

I am trying to implement an algorithm I created to get the sun rise, transit and set accurately, but in order to do that I need to get the Earth-Moon barycenter (EMB) perihelion, and not the Earth perihelion. I tried to get such time using the Horizons System, but it resulted in weird outputs (it showed that the perihelion is around December 1st, 2021, but it should be closer to the Earth perihelion at January 4th, 2022). Are there alternatives to get the EMB perihelion time?

• I am unable to reproduce your results. I found EMB perihelion to have occurred at 2022-Jan-03 20:16:08 TDB using Horizons. Tell us what points you selected, what outputs you selected, and how you determined when perihelion occurred. I chose the Earth Moon barycenter as one point, the center of the Sun as the other, and vectors as output. I then looked where range rate changed from negative to positive. Mar 20, 2022 at 17:30
• It appears you might have chosen the solar system barycenter rather than the center of the Sun as one of the two points of interest. Using Horizons, I am seeing a perihelion passage time of 2022-Jan-03 20:16:08 TDB but a pericenter passage time of 2021-Dec-03 00:44:09 TDB. Mar 20, 2022 at 17:54
• It's true, I used the solar system barycenter. How do I set it to the center of the Sun? I couldn't find the option anywhere. Mar 20, 2022 at 17:57
• You might be heading down the wrong path, you do not need to know the EMB perihelion to compute the sunrise and sunset. If the ephemeris you're using only gives you the Moon, and EMB, you need to compute the Earth position by multiplying by that vector by the Earth/Moon mass ratio, but you don't need it's parhelion. Predicting the sunrise/set to within 1 minute is nearly impossible due to atmospheric effects. You can get results within the margin of error, just by using the EMB as the Earth's position. Mar 21, 2022 at 14:29
• The Sun has the body id number 10 in Horizons. Mar 27, 2022 at 23:00