This word was coined by H. H. Turner.
par- (in parallax n.) + sec- (in second)
A unit of length equal to the distance at which a star would have a
heliocentric parallax of one second of arc, approximately equivalent
to 3·086 × 10^13 kilometres (19·2 × 1012 miles, 3·26 light years).
1913 F. W. Dyson in Monthly Notices Royal Astron. Soc. 73 342
There is ...
The earliest references that I can find about the measurement of helium D3 line is
D'Arrest, H. L. (1872). Über die Position der Lichtlinie D3 im Protuberanzspectrum. Astronomische Nachrichten, 79, 1. (On the position of D3 line in the prominence spectrum). For serious academic work, the 2 paged article need to be fully translated. You also need to see "...
He may simply not have cared. A reasonably small distance along the east-west axis will only lead to a very small error in the distance along the north-south axis.
E.g. an azimuth of 190° instead of 180° when looking from Alexandria to Syene will only lead to a 1.5% difference between straight distance and the north-south distance. This is probably ...