Supplement1 - Michel Fich Time Measurement SCI238 CEL...

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Supplement #1 Early Astronomy SCI238 CEL Supplement #1:The Sky The Origins ofAstronomy Sun pillar atTahoe © Michel Fich Supplement #1 Early Astronomy Time Measurement ! one second is now defined by a “hyperfine transition of cesium”… it has a frequency of 9,192.631,779 per second (ATOMIC TIME) ! one day was originally defined by the Sun’s position (e.g. noon to the next noon) and this defined the second: 24x60x60=86400 seconds = one synodic or solar day ) ( 10 8 . 1 1 second atomic one second solar one 8 years t ! " + = Definitions: sidereal time – time according to the stars synodic time – time according to the Sun (also called solar time) Each year is 0.57 seconds longer than then previous year! Supplement #1 Early Astronomy solar day vs. sidereal day it’s all in the orbit! Earth travels ~1 o /day around its orbit So…one solar day requires ~361 o of rotation …and a solar day is ~4 min longer than a sidereal day Supplement #1 Early Astronomy many kinds of day, year… measuring by the stars : one sidereal day = 23 hours, 56 min., 4.091 sec. which is 86164.091 seconds Apparent Solar Time = time measured by Sun’s position in the sky (a local time) Mean Solar Time = average length of Solar day Standard Time is the system where all places on the Earth in a 15 degree wide (in longitude) Time Zone use the same Mean Solar Time Sidereal Year (time for Earth to complete one orbit against the stars) is 365.6366 Mean Solar Days Tropical Year (time from one spring equinox to the next – i.e. against the Sun) is 365.242199 Mean Solar Days
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Supplement #1 Early Astronomy The Calendar ! synodic month: 29.53 days (12x29.53=354 days) ! sidereal month: 27.32 days ! Roman calendar: 13 th month every 3 rd year ! Julian calendar: 12 months,30 or 31 days/month " leap year every 4 th year (year is then 365 ! days) " started 1 January 45 BC " added 3 extra months to 46 BC " actual year = 11 min, 14 sec <365.25 days " by 1582 the calendar was 10 days “short” Supplement #1 Early Astronomy The Calendar ! The current calender: " the Gregorian calendar: 4 October 1582 was followed by 15 October 1582 (in Catholic countries) century years were not leap years unless divisible by 400 in England and North America 11 days were added in 1752 when is Washington’s birthday?? Napoleon’s victory at Austerlitz in 1805 Supplement #1 Early Astronomy Ancient Greek Astronomy ! planet = “wanderer” – there were 7 planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) ! Pythagoras (~560-480BC) ! Aristotle (384 - 322BC) ! Aristarchus of Samos (310-230BC) ! Eratosthenes (276-195BC) ! Hipparchus (~190-120BC) ! Ptolemy (~140AD) Supplement #1 Early Astronomy Ancient Greek Astronomy ! Philolaus: non-geocentric cosmology: Earth moves around central fire (not the Sun!) & “counter Earth” on opposite side of this fire ! Pythagoras: the “heavenly bodies” are spherical ! Aristotle " How can the Earth move? ! Geocentric Universe " Correct explanations of phases of Moon and of eclipses " Sun much more distant than the Moon " Earth spherical
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Supplement #1
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