Overplotting the position of the Venus transit

How to accurately plot the position of Venus as it transitted in front of the Sun as observed by SDO/AIA.

import matplotlib.pyplot as plt

import astropy.units as u
from astropy.coordinates import SkyCoord, solar_system_ephemeris

import sunpy.map
from sunpy.coordinates import get_body_heliographic_stonyhurst
from sunpy.net import Fido
from sunpy.net import attrs as a

Let’s download an image of the Venus transit.

result = Fido.search(a.Time('2012/06/06 04:07:25', '2012/06/06 04:07:35'),
                     a.Instrument.aia,
                     a.Wavelength(1600*u.angstrom))
files = Fido.fetch(result)
aiamap = sunpy.map.Map(files[0])

For this example, we require high-precision ephemeris information. The built-in ephemeris provided by astropy is not accurate enough. This call requires jplephem to be installed. This will also trigger a download of about ~10 MB.

solar_system_ephemeris.set('de432s')

Out:

<ScienceState solar_system_ephemeris: 'de432s'>

Now we get the position of Venus and convert it into the SDO/AIA coordinates. The apparent position of Venus accounts for the time it takes for light to travel from Venus to SDO.

venus = get_body_heliographic_stonyhurst('venus', aiamap.date, observer=aiamap.observer_coordinate)
venus_hpc = venus.transform_to(aiamap.coordinate_frame)

Out:

INFO: Apparent body location accounts for 144.14 seconds of light travel time [sunpy.coordinates.ephemeris]

Let’s crop the image with Venus at its center.

fov = 200 * u.arcsec
bottom_left = SkyCoord(venus_hpc.Tx - fov/2, venus_hpc.Ty - fov/2, frame=aiamap.coordinate_frame)
smap = aiamap.submap(bottom_left, width=fov, height=fov)

Let’s plot the results.

ax = plt.subplot(projection=smap)
smap.plot()
smap.draw_limb()
ax.grid(False)
ax.plot_coord(venus_hpc, 'x', color='deepskyblue', label='Venus')
plt.legend()

plt.show()