According to astrological lore

, Virgo is ruled by the planet Mercury. But several astrologers consider this to be an interim measure only, until Virgo's true ruler, planet
Vulcan, is finally discovered.
Also referred to as the "Ghost Planet," Vulcan is a celestial body that some sources believe to be the closest planet to the Sun, circling inside the orbit of Mercury. The belief in Vulcan originated in the Seventeenth Century due to certain irregularities in Mercury's motion. French mathematician and astronomer Urbain Jean Joseph Le Verrier (who discovered Neptune on the basis of his calculations) firmly believed in the existence of Vulcan, although the majority of later day scientists appear unconvinced that there is such a planet in that location.
"[C]ertain irregularities in Mercury's motion" most likely refers to an observed but unexplained (prior to 1915, that is) residual
perihelion precession of Mercury's orbit (for an explanation, see "Precession of planetary orbits" towards the end of the article).
The official search for Vulcan in that region was eventually abandoned in 1929 after a total solar eclipse, ...
If that statement has a typo (as I suspect it does) and the year is supposed to be "1919," this would be the solar eclipse that
Eddington observed on Príncipe, which observations provided powerful confirmation of
Einstein's General Relativity (GR). Among a host of other things, GR neatly explains the residual perihelion precession of Mercury's orbit through mass dilation (Mercury's effective gravitational mass, as "seen" by the Sun, varies with its position along its orbit, being heavier closer to the Sun where the planet's instantaneous velocity is greatest). This finding, being only around 90 years old, has, owing to its youth, escaped the attention of many astrologers who continue to rely on much older, and therefore inerrant, Babylonian and Egyptian teachings that compel them to continue seeking Vulcan.
Another theory that I have heard put forward to explain Vulcan's elusiveness is that it is in constant opposition to Earth, i.e. it is always behind the Sun as viewed from Earth. This suggestion ignores the fact that Earth's orbit is also slightly elliptical, and Vulcan would need to to obey
Kepler's Laws of Planetary Motion with considerable artistic licence.
... but there are those who maintain that Vulcan is in an etheric or non-materialized state and can only be seen in ultraviolet light or is hiding behind the Sun.
Wibble. Glubb.
Also, the assertion quoted in an earlier post (see the "
New dimensions of meaning" quote) about the primary importance of the names given to celestial objects raises some interesting possibilities of sympathetic magic, should Vulcan be found and so named: "
Physical traits: has pointy ears; smells of rubber and sometimes of boiling sulphur.
Special talents: can make obnoxious people faint with just a touch to the neck; can say "Kirk" without giggling.
Shortcomings: painfully logical; singularly unartistic; given to long periods of dormant placidity, broken by occasional hot eruptions of surprisingly destructive violence."
'Luthon64