The Equinox is an ancient prison ship originally used to contain the Zodiac Emperors, which later became their base.
The Equinox resembles a spacefaring cargo ship or ocean liner in its large, boxy shape. Its hull is a patchwork of old and new metal. Its lower and uppermost floors are prison levels, with engines in the middle and back, and the bridge in the extreme stern of the ship. Its hull is very durable, but it lacks energy shielding or any other weapons besides a set of torpedo tubes--without torpedoes.
During the Zodiac Wars thousands of years ago, the Equinox was constructed as a warship for Scorpio's fleets. However, upon capturing Empress Libra, he retrofitted it as a prison ship, trapping her, the others and their forces one by one. He then hid the ship in the distant reaches of the galaxy, and died, leaving his prisoners to rot.
Thousands of years later, a descendant of Sagittarius house named Ophiuchus began a quest to find the legendary ship and its occupants. When he finally found it, he hired a band of mercenaries known as the Windjammers to retrieve it, which they did, bringing it to Earth's Moon. He joined them and freed the Zodiac Emperors, who made the craft their base of operations. From there they began launching attacks at the Earth and its new Rangers.