This is shown in Linda Prestwidge's master table of contents for hard cover Horizon issues in the reference link below.

[2] The May 1977 issue contained an insert from the publisher, Rhett Austell, informing the subscribers that Horizon would become a monthly magazine in soft cover.

Horizon was not able to attract enough subscribers to maintain the luxury magazine devoted to the arts and history that had been envisioned by Thorndike and Parton.

The response from the subscribers to the lower quality of printing and binding and a new emphasis on current events was overwhelmingly negative, resulting in the sale of Horizon to Boone, Inc. a year later.

Contributors included Robert Joffrey, Alan Rich, Lanford Wilson, Ray Bradbury and Brendan Gill.