How to calculate distance from object to point on celestial sphere
I'd like to calculate the distance (in arcsec) from the object nearest to ra='08h55m10s' dec='-7d14m42s' to that point on the celestial sphere.
I am actually having trouble identifying the object. I have python code to run a cone search:
from astropy.coordinates import SkyCoord
from astroquery.irsa import Irsa
import astropy.units as u
loc = SkyCoord('08h55m10s',' -7d14m42s','icrs')
table = Irsa.query_region(coordinates=loc,catalog="wise_allsky_4band_p3as_psd", radius= 1 * u.arcminute)
print(table)This prints:
designation ra dec ... angle id
deg deg ... deg
------------------- ------- ------- ... ------------------ ---
J085507.72-071428.4 133.782 -7.241 ... 291.84348499999999 0
J085511.22-071522.2 133.797 -7.256 ... 155.64652000000001 1
J085508.65-071354.0 133.786 -7.232 ... 337.39116000000001 2
J085509.94-071534.8 133.791 -7.260 ... 180.87171799999999 3
J085511.28-071528.1 133.797 -7.258 ... 157.48249000000001 4
J085509.47-071502.9 133.789 -7.251 ... 200.548395 5
J085513.60-071440.0 133.807 -7.244 ... 87.949470000000005 6
J085510.83-071442.5 133.795 -7.245 ... 92.439341999999996 7
J085511.39-071427.5 133.797 -7.241 ... 55.110833 8
J085513.30-071420.4 133.805 -7.239 ... 66.332006000000007 9And this is where I am stumped. Which of these objects is nearest to the point? (I assume it must be the object in the zero index). And why? And how can I calculate the distance from the object to that point on the celestial sphere?
Could you explain how you identified the object closest to the coordinates?
see the formula in Rob Jeffries answer. It is the correct formula.
Ok this clears my misunderstanding. Just to clarify, the smallest angular distance returned by the function is $55.110833^{\circ}=198 398.999"$, is this correct?
No! Think about it. Your query asked for objects within 1 arc minute (60 arc seconds) of your given coordinates. The search found ten objects.
You can use
SkyCoord.separationto compute the separation between coordinates:c1 = SkyCoord('08h55m10s',' -7d14m42s', frame='icrs') # your coords
c2 = SkyCoord(133.782, -7.241, unit='deg', frame='icrs') # first object in table
sep = c1.separation(c2)
print c1
<SkyCoord (ICRS): ra=133.791666667 deg, dec=-7.245 deg>
print c2
<SkyCoord (ICRS): ra=133.782208333 deg, dec=-7.24123611111 deg>
print sep
0d00m36.3947sThe table you printed is truncated in between. If you enlarge your terminal emulator, you will be able to see a column called
dist, which is the distance between your input coordinates and the corresponding object in arcsecs:print table['dist']
dist
------------------
36.420772999999997
44.138542000000001
51.987606999999997
52.838003
49.937266999999999
22.372699000000001
53.621690000000001
12.375482
25.27176
53.707365000000003Then the nearest object is the one whose separation is:
print table['dist'].min()
12.375482Or equivalently, the one whose index is:
print table['dist'].argmin()
7The separation is calculated using the equation that @Rob Jeffries pointed out.
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Content dated before 7/24/2021 11:53 AM
David Hammen 7 years ago
Re *I assume it must be the object in the zero index*. That's a bad assumption. The object closest to your coordinates is the eight one (index=7) on your list, J085510.83-071442.5. That tool is doing a SQL query and the results are unsorted.