def earthsun_distance(unixtime, delta_t, numthreads):
R = solar_position(unixtime, <NUM_LIT:0>, <NUM_LIT:0>, <NUM_LIT:0>, <NUM_LIT:0>, <NUM_LIT:0>, delta_t,<EOL><NUM_LIT:0>, numthreads, esd=True)[<NUM_LIT:0>]<EOL>return R<EOL>
Calculates the distance from the earth to the sun using the
NREL SPA algorithm described in [1].

Parameters
----------
unixtime : numpy array
    Array of unix/epoch timestamps to calculate solar position for.
    Unixtime is the number of seconds since Jan. 1, 1970 00:00:00 UTC.
    A pandas.DatetimeIndex is easily converted using .astype(np.int64)/10**9
delta_t : float
    Difference between terrestrial time and UT. USNO has tables.
numthreads : int
    Number to threads to use for calculation (if using numba)

Returns
-------
R : array
    Earth-Sun distance in AU.

References
----------
[1] Reda, I., Andreas, A., 2003. Solar position algorithm for solar
radiation applications. Technical report: NREL/TP-560- 34302. Golden,
USA, http://www.nrel.gov.