An equatorial mount tilts one of its axes to match the Earth's, so the sky's apparent rotation becomes a single steady turn. Everything else about the design, the counterweights, the alignment routine and much of the price, follows from that one idea.
Why the tilt matters
The sky appears to rotate around the celestial pole once a day. Align an axis with that pole and following any object needs one motion at one constant rate, which a simple clock drive can supply. Without the tilt, tracking needs two motors at continuously changing speeds, which is a computer's job rather than a clockwork one.
Polar alignment, which is less painful than its reputation
For visual use, pointing the mount's polar axis at the pole star by eye is enough to keep an object in a medium power field for many minutes. For photography it needs to be right, and a polar scope built into the axis makes it a five minute job with a phone app supplying the current position of Polaris relative to the true pole.
German equatorial and its counterweights
The common form puts the tube on one end of a shaft and a counterweight on the other. It balances well and handles heavy tubes, and it has one quirk: as an object crosses the meridian the mount must be flipped to the other side, which interrupts a long exposure sequence and surprises every beginner the first time it happens.
Wedges and platforms
A wedge tilts an alt-azimuth fork mount until its axis points at the pole, converting it into an equatorial. An equatorial platform does the same job for a dobsonian: a driven tilted table the whole telescope sits on, tracking for an hour or so before it needs resetting. Both give equatorial behaviour without buying an equatorial mount at all.
Questions people ask about equatorial mount
What is an equatorial mount?
A mount with one axis aligned to the Earth's rotation axis, so a single constant motion follows the sky and keeps an object in the field of view.
Do I need an equatorial mount?
For visual observing, no. For long exposure astrophotography, effectively yes, or an alt-azimuth mount with a wedge or a platform doing the same job.
What is an equatorial wedge?
An adapter that tilts a fork mounted telescope so its axis points at the celestial pole, turning an alt-azimuth mount into an equatorial one for photography.