# First spectroscopic detection of Uranus' rotation?

As an undergraduate exercise quite a while ago, we placed the equator of Jupiter's disk across the slit of a high resolution grating spectrometer and then measured the tilt of the resulting line in order to estimate Jupiter's rotational velocity.

1. When was an experiment of this type first performed for Uranus?
2. What was the result? Did it make sense at the time? Was Uranus' 98 degree axis tilt appreciated?

Slipher 1912 and Lowell 1912 used a one-prism spectrograph on the 24-inch refractor at Lowell Observatory in this way. Their 1911 observations of Uranus yielded a period of 10.8 hours, which made sense given the polar flattening previously observed by others.

They knew the obliquity of Uranus's satellites' orbits and assumed that its equator was in a similar plane. This affected their observations in two ways. First the solstice of 1901-02 presented an unfavorable pole-on aspect; after a negative result in 1903, Lowell waited until 1909 to try again. Then Uranus required an unusual orientation of the spectrograph, so Slipher took special care to avoid instrument flexure bias.

Moore and Menzel 1930 credited Slipher and Lowell with the first such observation and obtained a similar result themselves.

• I've added a plot to supplement your answer. If you'd like to use the script to make a better plot (zoom in on some dates) please feel free! – uhoh Jul 30 '19 at 10:53

I've just plotted Uranus' axis direction versus the direction we view it at, to put those dates into context.

The "solstice of 1901-02" is clearly seen in the plot.

Plotted using Python script based on Skyfield:

import numpy as np
import matplotlib.pyplot as plt

halfpi, pi, twopi = [f*np.pi for f in (0.5, 1, 2)]

years    = np.arange(1800, 2100, 0.1)
times    = ts.utc(years, 1, 1)

Sun      = de423['sun']
Earth    = de423['earth']
Uranus   = de423['uranus barycenter']

Uranus_axis_RA_degs, Uranus_axis_Dec_degs = 257.311, -15.175 # https://link.springer.com/content/pdf/10.1007%2Fs10569-007-9072-y.pdf

axis       = Star(ra_hours    = Uranus_axis_RA_degs * 24. / 360.,
dec_degrees = -15.175)
Axis_obs   = Earth.at(times).observe(axis)
Uranus_obs = Earth.at(times).observe(Uranus)
angle      = Uranus_obs.separation_from(Axis_obs)

brk        = np.abs(RA._degrees[1:] - RA._degrees[:-1]) > 10.
RA, Dec    = RA._degrees[:-1], Dec._degrees[:-1]
RA[brk]    = np.nan

if True:
plt.figure()
plt.subplot(2, 1, 1)
plt.plot(RA, Dec)
plt.plot([Uranus_axis_RA_degs], [Uranus_axis_Dec_degs], 'ok')
plt.xlim(0, 360)
plt.ylim(-30, 30)
plt.xlabel('RA (degs)', fontsize=16)
plt.ylabel('Dec (degs)', fontsize=16)
plt.subplot(2, 1, 2)
plt.plot(years, angle.degrees)
plt.xlabel('Year', fontsize=16)
plt.ylabel('view vs axis (degs)', fontsize=16)
plt.suptitle('Uranus from Earth versus spin axis', fontsize=16)
plt.show()