is this Jupiter?

bfa1509 wrote: »

Would have thought that was Venus myself

No it’s jupiter

Andy From Sligo wrote: »

Took this this evening 22:45 13/7/2019.

been told this is Jupiter ?

Definitely has to be Jupiter, there's nothing else it could be. However, when I did a bit of amateur forensics on it, I found an anomaly I don't know how to reconcile. Here's my sketch of the number of moon diameters from the centre of the moon to Jupiter:





I took a line through the centre of both objects (thin orange arrow), then drew a moon diameter along it (thick dashed orange arrow), then copied and translated that (thick red arrows), to get almost exactly two moon diameters. In spite of being hand drawn over the original image I imagine it must be accurate to a few tens of percent at worst.

Here's Stellarium for the same date and time:





The moon phase is 91% and the features and orientation also match up pretty well to the photo, allowing for a bit of overexposure. The azimuthal grid lines are one degree apart. Even by eye you can see the separation between the centres is more than two moon diameters. Stellarium gives the positions in degrees to better than four decimal places. For small angles we can work out the separation just using Pythagoras theorem on the azimuth/altitude measurements. Stellarium also gives the moon apparent diameter at 0.51 degrees. Plugging in the numbers gives a separation of 3.3 moon diameters.

I don't see how the measurements from the photo could be out by this much. The altitudes are around 15 degrees, too high for the geometric vs. apparent positions to be affected by refraction. Just in case the timestamp on the photo is wrong (e.g. due to daylight savings) I looked at the positions for an hour earlier and later. The moon moves east along the ecliptic by around half a degree per hour, so could make a difference. But 21.45 is before sunset (especially in Sligo) and the separation is still 3.0 moon diameters. At 23.45 it increases to 3.7 moon diameters. If the date was wrong the moon phase would be very different, and there are no nearby planets. (Although flicking forward a day at a time brought me completely fortuitously to tomorrow's partial eclipse, which I hadn't known about ).

So what am I doing wrong?

I know the positions of asteroids need to be updated from time to time in Stellarium. I remember contacting them to point out that their location for Ceres was different from my photo. So they said to update with the most recent ephemeris from the MPC, and this sorted it. Perhaps the Moon's location also starts to wander, although I've never updated it myself and on the most recent occasion when I've used it for a star occultation the accuracy was acceptable.

Thanks for the last couple of suggestions.

tjhook, I mentioned I am not depending on the visual size or appearance of the Moon in Stellarium. What I did was use the Az/Alt coordinates. For instance, here are the values for 13-Jul 22:45 after converting Stellarium's coordinates to decimal degrees:



The hypotenuse is simply the square root of the sum of the squared differences. By rights I should scale the azimuth by the cosine of the altitude, as degrees in azimuth get "skinnier" with higher altitude. But as both bodies are ~15 degrees from the horizon the difference in the calculation is less than 2%. To convert to moon diameters I am using Stellarium's diameter value of 0.51 degrees, but there is no reason to doubt this as it would be typical for an average moon distance and doesn't vary by a large percentage anyway. Only after using the Stellarium data did I sanity check that the actual moon appearance in Stellarium is consistent with the data, but I am not depending on that.

SophieL, interesting point about ephemeris data within Stellarium. I wasn't sure how to sanity check it. You learn something new every day and while investigating, I encountered the difference between geocentric and topocentric moon ephemerides. Geocentric, as the name implies, are moon positions as measured from the centre of the earth. Topocentric moon positions are from the observer's location and can differ by a whole degree for two observers separated by an earth radius. In retrospect, this makes obvious sense considering the moon distance. I can only assume that Stellarium calculates its own topocentric moon ephemeris, as otherwise it would be ignoring the observer's location setting. I did check that my location is set for Ireland.

I also found what is presumably the gold standard for lunar ephemerides -- the 600-year NASA/JPL DE405 Ephemeris Model (sounds impressive anyway! :pac:). One calculator based on it can be found here. Using date/time of 2019-07-13 22:45:00 LT = UT+1, and lat/long of +53/-6.25 I got topocentric lunar coordinates that matched Stellarium to within a couple of arcminutes, in both Az/Alt and RA/DEC. It also confirms the lunar diameter of 0.51 degrees. I checked a second ephemeris at calsky.com and got the same results as Stellarium for both the moon and Jupiter.

So it looks like Stellarium checks out very well indeed. Does that cast doubt on the fidelity of the photo image? I don't know. I'm still stumped.

Andy from Sligo ... can you confirm that you took the photo in Sligo (or at least western Europe), and not on holliers in Asia or North America?

Andy From Sligo wrote: »

I can confirm I took the photo 12miles south of Sligo Town yes

And did you notice any other disturbances in the spacetime continuum, apart from the one that knocked the Moon off track by about 2,500 miles? :pac:

Just to check I'm not going nuts, I checked again in Stellarium specifically for 12 miles south of Sligo town. It affected the distance between the Moon and Jupiter by 45 arcseconds ... about a hundredth of a degree. Don't suppose there's any chance it was an aeroplane and not Jupiter at all? After all, the moon was 10,000 times brighter than Jupiter, so quite a contrast for a camera to capture. Next opportunity to capture the two of them close together ( < 2 degrees) is August 9th.

(P.S. Was cloudy at moonrise tonight but got a brief view of the eclipse sometime later).

Andy From Sligo wrote: »

I had to 'merge' with the help of paintshop

Uh ... right. What exactly did you 'merge', and how? :eek:


That original image is a whole different kettle of fish to the first one you posted. For starters, the moon is smaller and Jupiter is more like the expected 3 diameters away. And by matching up the light and dark patches in the diffuse streamers around the moon in the two images, I believe your 'merged' moon is the size of the red circle. I think my problem may be solved. :pac:

That was impressive