View Full Version : Discrepancies in visual magnitude given by software
According to my copy of Starry Night Backyard, Vesta's current magnitude is 5.76. However, Heaven's Above's list of top 10 current brightest minor planets lists it as 7.6. Clicking on Vesta and getting specific orbital data (still on Heaven's Above) lists it at 6.2.
So, which is the correct magnitude, and how are there such large errors?
2004-Sep-03, 11:56 AM
How do you calculate the magnitude of an asteroid? I presume you use a formula that combines the asteroid's albedo, diameter and distance? Perhaps three different formula's are being used?
Have you tried emailing their respective support services?
#-o Sorry, that's three more questions and no answers.
2004-Sep-03, 06:17 PM
Maybe they are taking into account the asteroid's light curve? It's a 5 hour rotation so there should be some variability in magnitude over <1 day. But 2 magnitudes sounds like an awful lot. :-k
FTR, my own software, which uses astorb.dat (http://www.naic.edu/~nolan/astorb.html) data, agrees with Starry Night.
Anyone with clear skies want to go out in the morning and check it visually? :lol:
5.76 seems wrong, though. That would make it naked eye visible, and I thought Vesta only ever got brighter than mag 6 when it was at perihelion and opposition. :-k
2004-Sep-04, 10:35 AM
It is pretty close to opposition now, although it's not at perihelion.
Incidentally, Redshift 4 gives its magnitude as 8.51! But it says that Vesta's distance is about 2.629 AU, whereas Starry Night says it's only 1.371 AU away.
Might some of these be using the asteroid's distance from the Sun rather than from the Earth?
2004-Sep-04, 07:29 PM
SkyMap (with an old database) = 6.2
Cartes Du Ciel using the currect soft06bright catalog = 6.1
It also lists solar distance as 2.3592 au, Earth distance as 1.3713 au.
The MPC generated ephemeris also estimates magnitudes of 6.2 - 6.1 over the next month. It would be interesting to find the reason for the magnitude discrepencies under certain software packages.
I took a look under very dark skies and wasn't able to see it naked eye, and unfortunately I didn't have my telescope with me so I can't compare it to the brightness of nearby stars.
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