The astrological signs are about a month out from the astronomical constellations. When an astrologer says the Sun is in Cancer, an astronomer will look for it in Gemini.
If Mars is in Virgo, astronomically it means that, if you look at the group of stars known to the Western cultures as Virgo, you will see an extra red star-like object amongst those stars. That will be Mars. However when an astrologer says that Mars in is Virgo, he is talking about the abstract astrological sign that has no physical significance. Mars will actually be in Leo. The difference in the two solar calendars is caused by an astronomical phenomenon called precession.
An astrologer would make predictions based on the position of Mars in an imaginary sign. An astronomer would be able to point out Mars; it would be found amongst the stars of Leo.
Even to say Mars is IN Leo is incorrect. Mars would be in the same direction as the stars of Leo. These are themselves at different distances. It is a bit like holding a hand against a building saying the hand is IN the the building.
Mars is said to be the god of war because its red colour reminded the ancients of blood. Mars is red because its surface is covered with iron oxides - rust! Also, the constellations in use by the West are not the same as those used by other cultures. The Chinese and Indians have totally different zodiac signs as well as different numbers of signs of the zodiac. Remember, the signs of the zodiac are created by humans and do not exist in the universe.
Astrologers invented the signs of the zodiac (as distinct from the constellations) after the effects of precession became generally known. If the constellations can be likened to the Earth's countries (France, USA, Namibia) then the signs of the Zodiac can be likened to ancient kingdoms that no longer exist (Babylonia, Ottoman Empire). Each Zodiac sign is 30 degrees wide in the sky. This is how astrologers have defined them. The astronomical constellations are all of different sizes, much like countries on the Earth.
Some astrologers will say that a particular zodiac sign represents the essence of a particular time of year, much like a seasonal effect. But each hemisphere of the Earth has different seasons at the same time of year. Some areas of the Earth have four seasons (Spring, Summer, Autumn, Winter) while other areas have two (Wet, Dry).
The stars do not cause the seasons. They are convenient markers for the apparent path of the Sun around the starry sky. I say apparent because the Sun's movement is really due to the Earth moving around the Sun. The seasons are caused by the Earth's equator being tilted by 23 and a half degrees from the plane of the Earth's orbit around the Sun.
On 21st June the Northern hemisphere passes through the Summer Solstice. 2000 years ago the Sun would have been in front of the stars that we call Cancer at this solstice. Hence the Tropic of Cancer. In fact at this solstice now the sun would be in Gemini. If a Total Eclipse of the Sun were to occur at the solstice, the stars of Gemini would be visible around or near the sun.