Equinox is the fifth album by Styx, released in December 1975.
The album was the band's first release for A&M Records (with whom they signed earlier in 1975, after the unexpected success of the single "Lady").
The album also marked the final appearance of original Styx guitarist John Curulewski who left the band  abruptly following the release of Equinox. The band went into a frantic search to find a replacement for their upcoming tour to support Equinox. Soon after, they found Tommy Shaw.
The album's biggest hit was the track "Lorelei", which was Styx's second US Top 30 hit. Another well-known song in the album was "Suite Madame Blue" (the title of which has a play on the word "sweet" to refer to the musical term of "suites", i.e. unrelated instrumental successions) which was written about the upcoming Bicentennial of the US. The track became a staple for all Styx tours with Dennis DeYoung. It's a rare example of a Styx song that (briefly) requires four voice parts; during the Return to Paradise tour, the "America" bridge would be sung by DeYoung, Shaw, James Young and the usually silent Chuck Panozzo.
Although Equinox stalled at #58, it went Gold in 1977 shortly before the release of The Grand Illusion.