KariRoad said:
Other than "apparent appearance" is it not rather The Earth which "enters" a Sign?
http://lasp.colorado.edu/~bagenal/1010/graphics/sun_in_zodiac.jpg
It does seem that "Aries Day One" is an agreed reference for the Vernal Equinox,
no matter if Astronomer and/or Astrologer, when establishing the First Day of Spring.
Or perhaps not?
http://singingsun.com/wp-images/Zodiacs.jpg
Anyway, I'm happy!
No the Earth never enters a sign. The reason is that the signs are human constructs, they have no existence or meaning independent of Earth and humans. Indeed the Tropical Zodiac cannot exist on any other planet than Earth as it takes it's definition from the seasons of the Earth.
The map shown in your 'colorado; link is wrong on at least two counts. First it shows the constellations, not the signs as can be deduced from the lines linking points on the 'zodiac'. Constellations are not signs, signs are not constellations. Secondly in it's marked points for the Sun, it shows the Sun seeming to change 'signs' on the 21st of each month, in fact this doesn't happen and it can be quite early such as the 19th in some months or quite late such as the 23rd or even 24th in other months. Of course if they know they are using constellations, those could just be markers but then the diagram becomes Astrologically irrelevant. There's at least one other possible issue, the point of the Vernal equinox in sidereal terms is much earlier in Pisces than shown on the diagram - assuming they think they are indicating signs rather than constellations.
Some misguided Astrologers do try and construct heliocentric charts, which allegedly show the positions of the planets from a solar point of view projected onto the 'zodiac' - they would argue that on their basis the Earth was in Cancer at the moment, the opposite sign to the Sun (which we 'see' in Capricorn). Some programs like Solar Fire allow such charts to be generated but if you cast two charts for the same time and place, one using the heliocentric setting and one using the geocentric setting you get the SAME Ascendant and the SAME MC and the SAME Houses, which shows that the Heliocentric chart also has no meaning other than on Earth and other than in Geocentric and human terms.
There's a possible argument that the Sidereal Zodiac could have meaning on, say, Mars but that would only be so if there were humans there and a functioning society very similar to that of Earth - a few hundred astronauts would not count. As the Sidreal Zodiac has very few adherents in the West, I think we might only see an Astrological issue if Mars is colonised by a few million people of Indian origin, who practice Vedic Astrology.
One thing that is more likely to have an impact on Astrological thought is if a child is born to human parents on the Moon, or Mars (which is the only other likely planet that might be have a base established on it. In that case it would be virtually impossible to produce a Western natal chart for that child!
PS, for children who might be born on space stations in geocentric orbit, it's possible to come up with latitude and longitude of birth.
Edited to add
A little bit of research shows that the Colorado map is intended for a course in Astronomy as such it is irrelevant to Astrology as it does use constellations and not signs. It is however, also Astronomically wrong because it leaves out Ophiuchus, which it should show, as the Sun does pass in front of that constellation, which of course is the point made in the original article.
First rule of Astrology - Signs are not Constellations
see
http://en.wikipedia.org/wiki/Zodiac
