Analemma

In astronomy, an analemma (/ˌænəˈlɛmə/; from Greek ἀνάλημμα "support") is a diagram showing the deviation of the Sun from its mean motion in the sky, as viewed from a fixed location on the Earth. Due to the Earth's axial tilt and orbital eccentricity, the Sun will not be in the same position in the sky at the same time every day. The north–south component of the analemma is the Sun's declination, and the east–west component is the equation of time. This diagram has the form of a slender figure-eight, and can often be found on globes of the Earth.

Diagrams of analemmas frequently carry marks that show the position of the Sun at various closely spaced dates throughout the year. Analemmas with date marks can be used for various practical purposes. Without date marks, they are of little use, except as decoration.

Analemmas (as they are known today) have been used in conjunction with sundials since the 18th century to convert between apparent and mean solar time. Prior to this, the term referred to any tool or method used in the construction of sundials.

Ptolemy

Claudius Ptolemy (/ˈtɒləmi/; Greek: Κλαύδιος Πτολεμαῖος, Klaúdios Ptolemaîos, [kláwdios ptolɛmɛ́ːos]; Latin: Claudius Ptolemaeus; c. AD 100 – c. 170) was a Greco-Egyptian writer, known as a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in the city of Alexandria in the Roman province of Egypt, wrote in Koine Greek, and held Roman citizenship. Beyond that, few reliable details of his life are known. His birthplace has been given as Ptolemais Hermiou in the Thebaid in an uncorroborated statement by the 14th-century astronomer Theodore Meliteniotes. This is a very late attestation, however, and there is no other reason to suppose that he ever lived anywhere else than Alexandria, where he died around AD 168.

Ptolemy was the author of several scientific treatises, three of which were of continuing importance to later Byzantine, Islamic and European science. The first is the astronomical treatise now known as the Almagest, although it was originally entitled the "Mathematical Treatise" (Μαθηματικὴ Σύνταξις, Mathēmatikē Syntaxis) and then known as the "Great Treatise" (Ἡ Μεγάλη Σύνταξις, Ē Megálē Syntaxis). The second is the Geography, which is a thorough discussion of the geographic knowledge of the Greco-Roman world. This manuscript was used by Christopher Columbus as the map for his westward-bound path to Asia, in which he discovered the hitherto unknown lands of the Americas. The third is the astrological treatise in which he attempted to adapt horoscopic astrology to the Aristotelian natural philosophy of his day. This is sometimes known as the Apotelesmatika (Ἀποτελεσματικά) but more commonly known as the Tetrabiblos from the Greek (Τετράβιβλος) meaning "Four Books" or by the Latin Quadripartitum.