<p>The information you need, which has included both historical measurements and more recent measurement with HST, is contained in <a href="https://arxiv.org/abs/1703.10625" rel="nofollow noreferrer">Bond et al. (2017)</a>. Their Table 4 lists the best-fitting astrometric orbital elements for the Sirius AB system.</p>
<p>The orbital period is <span class="math-container">$50.1284 \pm 0.00434$</span> years and the epoch of <em>peri</em>astron is <span class="math-container">$1994.5715 \pm 0.0058$</span>.</p>
<p>The epoch of apoastron, is  half a period on from this and
would be <span class="math-container">$2019.636 \pm 0.006$</span>, where I don't think further significant figures are warranted. However, this <em>does not</em> 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 <a href="https://www.stelledoppie.it/index2.php?iddoppia=27936" rel="nofollow noreferrer">double star database</a> suggests that the epoch of maximum separation on the sky is actually (I think) <span class="math-container">$2023.81$</span> and the uncertainty is likely to be similar to the uncertainty in apoastron. This is very close to TODAY (20th October 2023)!</p>

CC BY-SA 4.0
2023-10-20T13:01:13.813Z
That's what I have been looking for.  Thank you so much.
Steve8888
CC BY-SA 4.0
2023-10-20T13:30:19.280Z
@Steve8888 note edit.
ProfRob
CC BY-SA 4.0
2023-10-20T14:45:05.500Z
"2023.81" is that today?
Mitchell Porter
CC BY-SA 4.0
2023-10-20T15:52:49.357Z
@MitchellPorter Pretty much spot on (maybe 23 Oct - I think you would divide by 365.25).
ProfRob
CC BY-SA 4.0
2023-10-21T00:26:38.947Z
Shouldn't $2023.81$ be in 2024? I mean, however, that's $2023+0.81$.
Snack Exchange
CC BY-SA 4.0
2023-10-21T17:28:55.460Z
@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
James K
CC BY-SA 4.0
2023-10-20T12:18:03.220Z
2023-10-20T15:51:09.307Z
2023-10-20T15:51:09.307Z
ProfRob
<p>The only date that I can find for the <strong>latest apoastron of Sirius stars</strong> 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.</p>

CC BY-SA 4.0
2023-10-20T06:20:54.067Z
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?
ProfRob
CC BY-SA 4.0
2023-10-20T11:02:30.743Z
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
Steve8888
CC BY-SA 4.0
2023-10-20T12:50:47.137Z
See also https://astronomy.stackexchange.com/questions/28125/are-there-any-good-images-of-sirius-b-at-apastron-in-the-sirius-system
ProfRob
<star><orbit>
Latest apoastron of Sirius
Steve8888
# Latest apoastron of Sirius

(Asked by: username_0 on 2023-10-20)

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.

**Tags:** star, orbit
> **Comments:**
> **username_1**: 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?  
> **username_0**: 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  
> **username_1**: See also https://astronomy.stackexchange.com/questions/28125/are-there-any-good-images-of-sirius-b-at-apastron-in-the-sirius-system  

---

## 1 answers

### Answer by username_1 on 2023-10-20. Score: 6

The information you need, which has included both historical measurements and more recent measurement with HST, is contained in [Bond et al. (2017)](https://arxiv.org/abs/1703.10625). 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 *peri*astron 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](https://www.stelledoppie.it/index2.php?iddoppia=27936) 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)!

> **Comments:**
> **username_0**: That's what I have been looking for.  Thank you so much.  
> **username_1**: @Steve8888 note edit.  
> **username_2**: "2023.81" is that today?  
> **username_1**: @MitchellPorter Pretty much spot on (maybe 23 Oct - I think you would divide by 365.25).  
> **username_3**: Shouldn't $2023.81$ be in 2024? I mean, however, that's $2023+0.81$.  
> **username_4**: @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  

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