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Introduction to Astronomy ! AST0111-3 (Astronomía) ! ! ! ! ! ! ! ! ! ! ! ! Semester 2014B Prof

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Introduction to Astronomy ! AST0111-3 (Astronomía) ! ! ! ! ! ! ! ! ! ! ! ! Semester 2014B Prof. Thomas H. Puzia What is maximum/minimum observable altitude from Santiago for the following? ! RA DEC 33.4500° S Polaris 02h 32m +89d 15m 70.6667° W Crab Pulsar/ 05h 34m +22d 00m Nebula Large Magellanic 05h 23m -66d 45m Cloud (LMC) M42 (Orion 05h 32m -05d 23m Nebula) Southern Cross 12h 30m -60d 00m (Crux) Ω Cen 13h 26m -47d 28m Fomalhaut 22h 57m -29d 37m 4h 4h 0h 8h 20h 12h 16h N? What can you tell from this picture??? What time of day is it? Where is the Sun? Where would other planets lie? Where on Earth could you be? How do we know when a celestial object will transit? We define Local Sidereal Time (ST) to be 0 hrs when the vernal equinox (VE, which has RA=0) transits the observer's local meridian. One hour later, the local Hour Angle (HA) of the equinox is +1h (by the definition of Hour Angle), and the Local Sidereal Time is 1h. A star transiting now has RA~1h. At any instant, Local Sidereal Time = Local Hour Angle of the VE. Alternatively, Local Sidereal Time ~ Right Ascension of any star currently transiting (I use ~ here because 23h 56m does not = 24h) ! Now, LST is different from solar time, which defined as some variant of Greenwich Mean Time (GMT). To calculate the offset, roughly: Greenwich mean time of Noon March 21 => vernal equinox and Sun transit Greenwich together, RA=0 Each day, Sun position moves +3.94min in RA, providing an offset between solar time and local sidereal time. Your location on the Earth relative to Greenwich, UK is not the same as the median location of your time zone, so better to use your longitude (Santiago=70.6667° W), which is equivalent to 4.71111 hrs. Then the offset between local time and LST: LST - local time = 3.94 min * days past VE + (GMT/local diff - 4.7111h) Key Concepts: What are the different coordinate systems and why are each potentially useful? How do we use equatorial coordinates to gain a deeper understanding and intuition of the night sky? Theme • Time: Calendars Measures of time CALENDARS CALENDARS CALENDARS Importance of measuring the passage of time in various human civilizations. Example: Mayan calendar, very advanced. ! Historically we impose the calendar of the Roman Republic: Week: 7 days, one for each planet + sun + moon • Sábado Saturno • Domingo Sol • Lunes Luna • Martes Marte • Miércoles Mercurio FASTI • Jueves Júpiter • Viernes Venus ! Month: between 28 and 31 days, associated with the synodic period (phases of the Moon) ! Year: associated with the period of revolution of the Earth around the Sun and the repetition of seasons: 12 months = 52 weeks CALENDARS General consensus to divide years into months and days. Knowing the number of a day and the month's name can refer precisely to any day of the year. ! Difficulty: There are 365.2422 days in a year 12 months of 29.5 days do not make a year. An extra month was added every few years because of the gap. If we take 365 days in a year there is a lag of 0.2422 days per year. After 100 years would have 24 days of lag. Julian Calendar Early Roman calendar: 12 months of ~29 or 31 days, ends in February, occasional extra month of 27 days Julius Caesar (in 46 BC) tried fix things by adopting the strategy of the Alexadrian astronomer Sosigenes: ! It was known then that the solar/tropical year lasted 365.25 days ! Established a convention that there would be three consecutive years of 365 days, then one year of 366 days ! Every 4th year, an extra day is added to February: Leap year. “last year of ! Realign so year begins on Jan. 1 (46 BC was 445 days long!) confusion” Greatly reduced the problem; now only a difference of ~1 day in 100 years. Julius Caesar died in 44 BC: July named in honor of birth month August renamed in 8 BC to honor Augustus Caesar. This calendar worked well up to 1582, by which point there was an appreciable discrepancy between the equinox, spring and Easter. Gregorian Calendar " The discrepancy between the Julian and Solar year (365.242199 d) is 11m14s. By 1582 this had amounted to 10 days. " Pope Gregorio XIII tried to improve the situation by: # Abolishing October 5-14, 1582. # Proposing to skip 3 days every four centuries. " In his reformed calendar, years that end in 00’s (e.g. 1900, 2000) skip leap years unless they are divisible by 400. " This corrected the calendar to within ~1 day in 3300 years (1 yr = 365.2425 d) " Catholic countries adopted this immediately, but Protestant and other countries did not until later epochs. From 1582 to 1923, dual dates were often listed to avoid confusion (Julian and Gregorian). " In 1800s, Herschel proposed skipping leap years in 4000, 8000, etc. Such a calendar would only lose ~1 day only every 20000 years. Gregorian Calendar Julian Day Sometimes it is necessary to express the instant of an observation as a certain number of days plus some fraction of some fundamental time. Astronomer J. J. Scaliger chose noon of January 1, 4731 BC. The number of days from that date is the Julian Day. Important: Each new day begins at 12h00m Julian GMT (UT), half- day gap with the calendar day. Example: the Julian day 2444606 began at noon on January 1, 1981. It is common to use the Julian calendar for astronomical events. Universal Time GMT = Greenwich Mean Time, UT = Universal Time " Mean Solar time as observed from the meridian of Greenwich, UK (longitude = 0). Established in 1685, although disputed for 200+ yrs with Paris’ (and Belgium) meridian. " A countries’ local time is related to GMT and its time zone (e.g., UTC/GMT-4). " So time in a place refers to a time zone (can be somewhat arbitrary => up to 2 hrs “off”). Daylight Savings (summer time) " Controverisal (and complex) shift time of official “noon” to exploit sunlight after working hours. " Good: ~0.5% energy savings?, retailers, sports; Bad: farming, confusion. " Often attributed to Ben Franklin (satirically suggests Parisians rise early to conserve candles). " But, precise schedules not really required until rail and communication forced modern standardization of time (>1900s). " Modern version suggested by Hudson in NZ (also Willett in UK). However, not until WWI (1918) Ephemeris Time (ET) • UT and (sidereal time) ST are related to the period of rotation of the Earth. • But this period is not constant. It shows irregularities on “short” time scales of order seconds to minutes, and is slowing down on longer time scales due to various factors. •For example, the day was lengthened 1 / 2000 sec per 100 years (0.000005 sec / yr) due to the gravitational action of the moon. ET-UT=51 sec (Jan. 1900 vs Jan. 1980) •Astronomers need to measure time evenly. Ephemeris Time (ET) used from 1952 to ~1970, but phased out by atomic clocks. •The ET is calculated by the motion of the Moon, which is assumed uniform. International Atomic Time (TAI) • Atomic clocks use electronic transition frequency as unit of time. • Current standard for civil timekeeping. Basis for Terrestrial Time (TT) and Coordinated Universal Time (UTC) systems • TAI is a weighted average of the time kept by over 200 atomic clocks in ~70 national laboratories worldwide, compared using satellites. • The majority of the clocks are caesium clocks (9192631770 cycles) • Definition of the International Second written in terms of caesium Leap Seconds Clocks throughout history ~3500 BC ~350 BC ~2000 BC ~1200 AD ~2009 AD ~1960 AD Measuring Time (and rotation) " The period of the common pendulum depends on the mass and longitude: can be used to measure time " Foucault’s Pendulum " Jean Foucault in 1751 suspended a mass of 25 kg from a 25m cord in the Pantheon of Paris " The pendulum made marks in the sand, demonstrating that its plane of oscillation was not permanent " Historically important " Affected by the rotation of the Earth " Can define the latitude of a place " Period of revolution P = 23h56m / sinΦ" " Foucault’s Pendulum at the Pole " Foucault’s Pendulum at the Ecuador Measuring Time " Using the Sun and Earth as in ancient times is not so useful due to chaotic effects. We need very precise clocks today (e.g., GPS, guidance/control systems, synchronization, computing). " Example: pulsars have rotation periods of milliseconds. Clock (Time system) Precision Sun (UT), Stars (ST) minutes ET-UT=51 sec Pendulum seconds (Jan. 1900 vs Jan. 1980) Mechanical 1s/yr Quartz 1s/10yrs Moon (ET) 1s/300yrs Atomic Cs (SI sec) 1s/6,000yrs Atomic H Maser (TAI) 1s/100,000yrs Quantum Clock 1s/3,700,000,000yrs Measuring Time It is one thing to measure a time interval accurately, but another to measure long time intervals in the past. ! Modern science has developed several methods and accurate measurement of time spent: ! Biology: tree rings Physical-chemical: C14 radioisotope decay Astronomy: stellar evolution Key Concepts: History and Mechanics of calendars/time (origin, format, etc.). How do we “keep” time? Accuracy and importance of time-keeping Why do we “keep” time? Theme COSMOLOGY geocentric and heliocentric ! • Greek Astronomy • Ptolemy • Copernicus • Brahe • Galileo • Kepler • Kepler's laws • Newton • Newton's Laws • Orbits, satellites A historical prespective segwaying into physics Greek Astronomy " School in Alexandria (after Aristoteles) ! " Aristarchus of Samos (310-230 BC): " Explained the phases of the Moon " Tried to measure the distance to the Sun and Moon " Believed that the Sun was the center of the Universe " Eratosthenes (276-196 BC): " Measured the circumference of the Earth using the Sun’s shadow as seen from Alexandria and Asuan.
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