A gnomon, from Ancient Greek γνώμων [gnṓmōn]), meaning ’one that knows or examines’, is the part of a sundial that casts a shadow. The term is used for a variety of purposes in mathematics and other fields, normally being a stick-shaped object that is projected (by light beams) onto a measuring surface, and typically to measure directions, position, or time.
Plumb-Line
A plumb bob level, also known as a plummet, is a tool used to establish a vertical line, or plumb line. It’s made of a weight attached to a string, and is used to check vertical alignment.
Principles
- Hold the plumb bob above the area you want to measure.
- Let the weight drop down.
- The string will point in a vertical direction.
Applications
- Construction: Used to check vertical alignment in construction.
- Surveying: Used to find the local vertical direction, or nadir.
- Tools: Used in tools like steel tape and theodolites.
History
- The plumb bob dates to Ancient Egypt, and was invented over 4,000 years ago.
- It was a precursor to the spirit level, is used to check horizontal alignment.
Perspective Research Centre 
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