Time “Does anybody really know what time it is? Does anybody really care?” – Chicago

The Sky’s Natural Cycles Early cultures noticed 3 obvious periodic intervals: –day: seconds on average between each noon –month: days between each full moon –tropical year: days between June solstices Problem: None of these numbers divide evenly into one another. –How to construct a calendar? …with great difficulty Stonehenge

A modern solution to our complicated calendar? Alliance, Nebraska

Stonehenge: 3100 BC – 1600 BC

Timekeeping Devices s: Pendulum clock C. Huygens presenting pendulum clock to Louis XIV Accuracy: ~ 10 sec/day

Timekeeping Devices 1940s: quartz oscillator –use piezoelectric effect to drive vibrations in a quartz crystal using electric current from an oscillator circuit Quartz “tuning fork” with oscillation freq = Hz Accuracy: < 1 sec/day

Timekeeping Devices 1950s: atomic clocks: –fill a microwave cavity with a pure gas (cesium, rubidium, hydrogen) that has a hyperfine energy level structure –gas emits hyperfine transition microwaves, cavity is tuned for resonance by being connected to an electronic oscillator feedback circuit Modern chip- scale atomic clocks Accuracy: < sec/day

Local Apparent Solar Time Equal to the hour angle of the Sun + 12 h Why are sundials generally lousy timekeepers?

Mean Time Introduce a ‘mean’ Sun that moves along the celestial equator (not the ecliptic) at a fixed rate. Determine this rate by using the ICRS as a reference frame. Call the local mean time at 0° longitude (Greenwich) ‘Universal Time’ (UT1) –UT1 is the preferred time system for astronomers

Modern (SI) Definition of Time In pendulum clock era, 1 second was 1/86400 of the mean solar day, as measured by transit instruments –but the mean solar day is constantly changing, due to variations in Earth’s spin and its orbit around the Sun 1967: by definition, one second is the duration of 9 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of a cesium 133 atom at rest at absolute zero. 1967: by definition, one second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of a cesium 133 atom at rest at absolute zero.

Time for Some Complications The SI second (based on physics) is totally independent of the second that is 1/86400 of a mean solar day (based on astronomy) The world requires a uniform, unchanging time standard, but also needs it to be roughly in sync with day/night Astronomers need to be able to point telescopes and accurately time extraterrestrial events TAI = international atomic time (physics, SI seconds) UT1 = Universal time (astronomy, non-SI seconds)

UTC = Coordinated Universal Time What your watch/GPS uses Administered by international standards organizations Runs in step with TAI (uses SI seconds), but with a prescribed offset to keep it within one second of UT1 The offset is periodically changed by introducing ‘leap seconds’ (which may be abolished in the near future)

Standard (Zone) Time Invention of railways and the telegraph led to the adoption of standard time in 1884: – world is divided into 24 major time zones –standard time (offset from UTC) is the same everywhere within a time zone Consequence: the Sun is not usually due South when your watch reads noon. exact time of solar transit depends on your longitude, time zone, day of year, and small variations in the Earth’s rotation

Each time zone spans roughly 15º of longitude

Complications Not all countries like the time zone scheme (China). Some use daylight saving time: –move clocks back 1 hr in late Fall –start/end dates of DST different in Europe and N. America Lafayette uses Eastern Daylight Time from spring through fall.

International Date Line Where the calendar day begins. what is the date and time in Japan right now? 14 hours ahead of EST

Our Calendar

The Year Solar year: length of time for the Sun to return to the same equinox. Tropical year: length of time for the mean Sun to travel exactly 360° along the ecliptic Sidereal year: length of time for Earth to return to same spot in its orbit w.r.t. ICRS (20 min longer than tropical year)

Where our calendar comes from Julian calendar developed circa 200 AD, Rome –each year had 365 days and 12 months – every fourth “leap” year had 366 days Julian year ( days) slightly longer than tropical year ( days) –equinoxes and solstice dates migrated over time. –by the 16th century, March equinox had slipped back to Mar. 11

The Gregorian Calendar 1582: Pope Gregory XIII institutes major reform: –ten days were dropped: Oct 5, 1582 became Oct 15. –a century year was only a leap year if divisible by 400. The year 2000 was a leap year, but 1900 was not. –part of the infamous ‘Y2K’ problem –cost the world ~$400 billion to change software code –on Jan 1, 2000, the U.S. Naval Observatory master clock website read 1 Jan Worldwide standard is now Gregorian Calendar.

Dates in the Modern Era Modern dates contain A.D. (Anno Domini) –Also used: C.E. (Common Era) System invented by D. Exiguus in 547 A.D. –reckoned Jesus Christ was born 754 years after Rome was founded Dates before 1 A.D. are B.C. (Before Christ) There was no year zero.

Julian Dates Many areas in astronomy require specification of a specific date –e.g. variable stars, planetary orbits Calendar dates are a problem because of leap years, non- uniform historical international adoption of Gregorian calendar (some as late as 1922: USSR, 1923: Greece) The Julian date is the integer number of days since Jan 1, 4713 B.C. according to Julian calendar. ‘modified’ Julian dates often used: MJD = JD (MJD begins at midnight UT, JD begins at noon UT) –0h0m on Sept 9, 2015 = JD = MJD –plan your big astronomy bash: Feb 24, 2023 (MJD !!)

Coordinate Epochs Because of precession of the celestial equator on the sky, astronomical coordinates are always accompanied by an epoch. B1950: Coordinates on Jan 1, 1950, based on Besselian (tropical) year of … days J2000: Coordinates on Jan 1, 2000, based on Julian year (exactly days) To calculate coordinates on a given date, must apply a spherical transformation based on Earth’s axial precession parameters.