What field studies ancient monument alignments such as summer-solstice sunrise or sunset lines?

The field that studies ancient monument alignments such as summer-solstice sunrise or sunset lines is archaeoastronomy.

Archaeoastronomy examines how past societies understood and used the sky, combining archaeology with astronomy, anthropology, and the study of ancient calendars. Researchers investigate buildings, tombs, ceremonial landscapes, and other structures for deliberate relationships with the Sun, Moon, stars, or significant horizon events.

Summer-solstice alignments are especially notable because the Sun reaches its greatest seasonal declination then. At sites with a carefully chosen horizon or architectural opening, the sunrise or sunset can appear along a particular axis. Such alignments may indicate ritual importance, timekeeping, seasonal planning, or symbolic connections between the community and the heavens.

A common mix-up is treating every apparent alignment as proof of astronomical intent. Archaeoastronomers must consider construction accuracy, the surrounding landscape, later rebuilding, and the possibility that a pattern occurred by chance. The field therefore studies evidence and cultural context rather than simply labeling old monuments as observatories.