He has journeyed to 11 solar eclipses on land and on sea. His 1973 radio-photos from the Voyage to Darkness eclipse cruise ship Canberra which was positioned off Mauritania on the African coast appeared on the front page of the Times of London. A photo sequence of the 1979 eclipse near Winnipeg, Canada, was on the front page of The New York Times.

He took early retirement from the AP in 1993 to form his own photo agency, Feature Photo Service, which rents time on the AP satellite to transmit business and corporate photos to 1,000 newspapers across the United States and internationally via the internet.

He started with the AP in New York as a picture editor in 1965. In 1967 he moved to Washington during the tumultuous Watergate era, and was promoted to Boston in 1972 as New England Newsphoto editor, with responsibility for six northeast states. His aerial photo of the sinking of the Argo Merchant off Nantucket in 1976 was nominated for a Pulitzer Prize.

In 1978 he moved to Chicago as Assistant Chief of Bureau/Photos with a territory of 18 states and a staff of 26 photographers and eight photo editors. Assignments included the World Series, Super Bowls, the Pope's trip, and many of the top news stories of the day. In 1985 he helped to develop a graphics system for maps and charts using the Macintosh computer that became a service called AP GraphicsNet.

In 1989 Horwitz was appointed as AP's European Network Director. Based in London, he introduced photo and graphics computers into the European system. He returned to New York in 1991 as Senior Photo Editor to work on the new digital PhotoStream system that ushered in the new age of digital photography and photo transmission. His final project was PhotoExpress, which allowed outside agencies to rent time on the AP satellite. Upon his retirement he became its first commercial client.

His interest in astronomy began in high school with weekly visits to the Adler Planetarium in Chicago. He studied astrophysics at Indiana University, but photography captured his interest and he switched to photojournalism. He received a master's degree in Audio-Visual Communications and began his AP career in New York.

Among his eclipse adventures, he served as the photo lecturer for the Adler Planetarium's Mexico eclipse expedition in 1991.

During the Eclipse '99 — Black Sea Voyage to Darkness aboard the World Renaissance Mr. Horwitz shared with passengers, valuable techniques for eclipse photography and conducted sessions for those with questions about equipment.