The spectrum is made along the equator of Jupiter (Slit aligned to the equator, look at image below).
You can see the terrestrially atmosphere lines exact vertically, but the Doppler shifted lines from sunlight reflected on Jupiter are slantwise.
Over the difference of wave length it is possible to calculate the rotational speed of Jupiter and with the known rotational periode of 9 hours 50 minutes and 30 seconds we can calculate the diameter.
The diameter calculated on the sharp Ni 6643.64A line is 139184km, this is only 2.7% smaller than the real value of 142984km (wikipedia).
Telescope: Officina Stellare RC360AS, f/8, d=360mm
Spectrograph: Lhires III, 2400gr, slit 23u
Camera: SBIG ST10-Xe
Resolution: 0.089 A / pixel, resolving power ~ 17000
Date: 17. March 2015, UTC 21:13 - 21:21
processed with BASS project Calibrated with known spectral lines
6 images (6 x 60s) Dark corrected, cropped, and stacked with PixInsight