# Latest apoastron of Sirius

The only date that I can find for the latest apoastron of Sirius stars is 2019. Is there a more detailed date with at least the month and/or day. Obviously, it happened in 2019 but can't find any details. The date that I see in Wikipedia is a calculated date based on two and one-half 50.09 year orbits following the periastron epoch of 1894.13 giving a date of 2019.34. This calculation assumes the orbital period of 50.09. I'm more interested in the actual recent observation/calculation that perhaps was done in 2019. Thanks.

• I'm puzzled. The apastron is not measured, it is inferred by fitting an orbit and then calculating when the maximum separation would be. That date is 2019.34. Are you simply interested in confirmation that this is the most up to date reliable orbit for Sirius or in knowing what the error bar on that number is? Oct 20, 2023 at 6:20
• I'm sorry if I use the wrong terms since Astronomy is not my field. I'm interested in a more reliable date. The date of 2019.34 is a calculated date with more than 1% error. They used the orbital period of 50.09 which in itself is not exact and we know that it is close to 50 but with a +-. Anyway, they used the periastron date of 1894.13 and added half of 50.09 to reach the apastron for that time period. Next, they added (50.09 x 2) to get to 2019.34. (1894.13 + (50.09/2) + (50.09 x 2) = 2019.35 Oct 20, 2023 at 11:02
• Oct 20, 2023 at 12:50

The information you need, which has included both historical measurements and more recent measurement with HST, is contained in Bond et al. (2017). Their Table 4 lists the best-fitting astrometric orbital elements for the Sirius AB system.

The orbital period is $$50.1284 \pm 0.00434$$ years and the epoch of periastron is $$1994.5715 \pm 0.0058$$.

The epoch of apoastron, is half a period on from this and would be $$2019.636 \pm 0.006$$, where I don't think further significant figures are warranted. However, this does not correspond to the maximum separation on the sky bcause of the inclination of the orbit to the plane of the sky and its eccentricity. The double star database suggests that the epoch of maximum separation on the sky is actually (I think) $$2023.81$$ and the uncertainty is likely to be similar to the uncertainty in apoastron. This is very close to TODAY (20th October 2023)!

• That's what I have been looking for. Thank you so much. Oct 20, 2023 at 13:01
• @Steve8888 note edit. Oct 20, 2023 at 13:30
• "2023.81" is that today? Oct 20, 2023 at 14:45
• @MitchellPorter Pretty much spot on (maybe 23 Oct - I think you would divide by 365.25). Oct 20, 2023 at 15:52
• @SnackExchange No 2023.0 means the start of 2023 (Jan 1st), 2023.5 means the middle of 2023 (Jul 2nd) and 2023.81 means 23rd of October 2023 Oct 21, 2023 at 17:28