Time, coordinates and how the Sun and Moon move in the sky

Transcription

1 Time, coordinates and how the Sun and Moon move in the sky

2 Using the colors and magnitudes of quasars drawn from the SDSS Catalog Archive Server to distinguish quasars from stars using the light they emit Studying the motions of the Sun and the Moon in the night sky in order to predict eclipse times We ll need to know about times and time-zones Last Time... This Time... East-West and North-South coordinates How the Sun and Moon appear to move East-West and North-South over a day, a month, a year etc.

3 Precise timekeeping and MJD Given different time systems, leap years etc. it is useful to have a calendar with which to express exact times of events in the sky (referred to as epochs) In astronomy we use a calendar based on the original Julian calendar (established by Julius Caesar) Julian Date (JD) is a count forward in days staring from 0 at noon on January the 1st in the year (4713 BCE) Modified Julian Date (MJD) is a count in days from 0 midnight on November 17 in the year 1858 MJD = JD The YYYY-MM-DD date you re used to is called the ISO date (it s set by the International Standardization Organization)

4 Time zones MST (MDT is 1 hour closer to GMT) Prime Meridian Greenwich Mean Time (GMT)

5 Coordinates on the Sphere Any position on the surface of a sphere (such as the Earth or the night sky) can be expressed in terms of the angular coordinates latitude and longitude Equator latitude (δ) Latitude runs from -90 o to 90 o longitude (α) -90 o is a sphere s south pole (South Pole on Earth, South Celestial Pole in the sky) 90 o is a sphere s north pole (North Pole on Earth, North Celestial Pole in the sky) 0 o is a sphere s equator (the Equator on Earth, the Celestial Equator in the sky)

6 The Equatorial Coordinate System Longitude runs from 0 o to 360 o (it s chosen to instead be -180 o to 180 o on the Earth) In this equatorial coordinate system used in astronomy latitude (δ) longitude is called RA or Right Ascension and latitude is called Dec or Declination Equator longitude (α) We will use these coordinates. If you like, think of them as simply a pair of coordinates that specify a point in the sky We will always express these coordinates in degrees... note that an arcminute is one-sixtieth (1/60) of a degree

7 North-South & East-West Motions Right Ascension and Declination are measured relative to a fixed system of very distant so motionless stars The Earth spins and orbits relative to an axis along it s North to South Pole Because of this, the Sun s Right Ascension (longitude) and Declination (latitude) are affected by different things The Sun s RA (East-West position compared to distant, fixed background stars) is affected by East-West motions The Earth orbiting the Sun The Sun s Dec is affected by North-South motions Unless you walk north or south around the Earth, the only external effect on the Sun s Dec is the seasons

8 Sun s East-West Motion looking down on the Earth s North Pole The other star is up during the day and hidden by the Sun ~September 21 This star is visible at night RA=180 o This star is visible at night ~March 21 RA=0 o Looking down on Earth s North Pole, Earth s rotation and orbit are counter-clockwise RA=180 o RA=0 o The other star is up during the day and hidden by the Sun The position of the Sun compared to the fixed stars in the sky moves 360 o per year

9 What Causes the Seasons? It s the tilt of the Earth s rotation axis (North-South Pole line) to the Earth s orbital plane the Earth s orbital plane is called the ecliptic Note, the tilt direction doesn t change over the year

10 Approximate Solstices & Equinoxes in the Earth s Northern Hemisphere Winter Solstice December 21 night longer than day in the Northern Hemisphere Vernal (Spring) Equinox March hours of daylight, 12 hours of night Summer Solstice June 21 day longer than night in the Northern Hemisphere Autumnal (Fall) Equinox September hours of daylight, 12 hours of night

11 Sun s East-West Motion: Thoughts The tilt of the Earth to its orbital plane is ~23.5 o (this is formally called Earth s axial tilt) The equator is at 0 o latitude on Earth (which is the same thing as 0 o declination in the sky) So, the Sun moves from o declination (23.5 o N, the Summer Solstice in the Northern Hemisphere) to o (23.5 o S, the Winter Solstice in the Northern Hemisphere) Casper is near 43 o geographic latitude, which is the same thing as a line of 43 o declination in the sky

12 Reminder: Basics of Eclipses Lunar Eclipse When the Moon passes into the Earth's shadow Sun Earth Moon Full Moon Solar Eclipse When the Earth crosses the Moon's shadow Sun Moon Earth New Moon Why don t eclipses happen every Full and New Moon?

13 The Moon s Motion Relative to the Sun The Moon s orbit is tilted by 5 º to the Earth s orbital plane (which, remember is called the ecliptic) The Moon appears to be about ½ o in angular size in the sky, so 5 o is ~10 Moon diameters The Moon must also be near the ecliptic for an eclipse to occur (once each month!)

14 Time, coordinates and how the Sun and Moon move in the sky