When the activated ability resolves, determine whether the targeted spell would destroy a land if it resolved right then. If it would, then counter that spell. Otherwise, it is not countered, even if the spell could, under other circumstances, destroy a land.

Will not counter a spell which would destroy a land only if a choice is made.

Equinox will not counter a spell which would indirectly cause destruction of one of your lands.

Equinox will not counter a spell which requires sacrificing land when it enters or one that requires sacrificing land as part of the cost to cast it.

Equinox will not counter a spell that deals damage to an animated land, even if it would deal more damage than the land's toughness. This is because the spell itself does not destroy the land directly. The land is destroyed by a game rule.

The ability can target any spell, even one that would not destroy a land.

Equinox will not counter a spell that has a random chance of destroying a land.

Equinox : règles, fonctionnement et interactions (MTG)

Equinox is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Enchant land Enchanted land has ": Counter target spell if it would destroy a land you control."

How it works : Equinox

Original English text — Wizards of the Coast / Scryfall

#1September 20, 2013· Wizards of the Coast
When the activated ability resolves, determine whether the targeted spell would destroy a land if it resolved right then. If it would, then counter that spell. Otherwise, it is not countered, even if the spell could, under other circumstances, destroy a land.
No other card is named in this ruling.
#2October 1, 2008· Wizards of the Coast
Will not counter a spell which would destroy a land only if a choice is made.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
Equinox will not counter a spell which would indirectly cause destruction of one of your lands.
No other card is named in this ruling.
#4October 4, 2004· Wizards of the Coast
Equinox will not counter a spell which requires sacrificing land when it enters or one that requires sacrificing land as part of the cost to cast it.
No other card is named in this ruling.
#5October 4, 2004· Wizards of the Coast
Equinox will not counter a spell that deals damage to an animated land, even if it would deal more damage than the land's toughness. This is because the spell itself does not destroy the land directly. The land is destroyed by a game rule.
No other card is named in this ruling.
#6October 4, 2004· Wizards of the Coast
The ability can target any spell, even one that would not destroy a land.
No other card is named in this ruling.
#7October 4, 2004· Wizards of the Coast
Equinox will not counter a spell that has a random chance of destroying a land.
No other card is named in this ruling.