Night Sky Chapter 4
Topics • The celestial sphere • Motion of the Sun • Phases of the Moon • Motion of the stars • Constellations and the changing view of the night sky
Remember! • Although we see very nice pictures and diagrams of planets circling the Sun, the Moon circling the Earth, etc., we only observe things from the Earth (or near the Earth in the case of space-based telescopes). • Our only reference frame for observation is that of the Earth (except for the few space probes, telescopes, etc.) • The positions of stars (because they are so distant) are essentially fixed. We are the ones who are revolving (around the Sun) and rotating (about our axis). Thus, in our reference frame, stars appear to move.
Celestial coordinates • How do we specify the position of something in the sky? • “over yonder” is not technical enough. • consider how we specify objects on the earth • longitude (gives E/W location relative to the Prime Meridian) • latitude (gives N/S location relative to the Equator) • use a similar coordinate system on a Celestial sphere
NCP VE SS WS AE SCP Celestial sphere • Aligned with the Earth • The Celestial Poles • Celestial Equator • The Ecliptic • Vernal Equinox • Autumnal Equinox • Winter Solstice • Summer Solstice
NCP VE SS WS AE SCP Location of stars • Right ascension is measured ccw from the vernal equinox in units of hr:min:sec • Declination is measured from the celestial equator in deg:min:sec
Question • What is the radius of the celestial sphere? • infinity • at about the Moon’s distance from Earth • at about the Sun’s distance from Earth • at about the nearest star’s distance from Earth • none of the above • Answer: E
Example NCP , 90 degrees • star location • RA: 10 hrs • Dec: 75 degrees VE 0 hours SS 6 hrs 0 deg WS 18 hrs AE 12 hrs
Question • Does a star’s right ascension and declination depend on where you are located on the Earth when you observe the star? • yes • no • yes and no because it depends on your latitude, but not your longitude • yes and no because it depends on your longitude, but not your latitutude • Answer: B
Precession • Axis wobbles • Period is 23,000 years • “North Star” is not always Polaris • Locations of all stars as measured by right ascension and declination will change
Azimuth is the angle around from north and Altitude is the angle up from the horizon. Bean-boy sees the object at about azimuth 200 deg and altitude 60 deg above horizon. Zenith is the point directly overhead S 180 deg E 90 deg W 270 deg N 0 deg Local sky coordinates 90 deg, Zenith
Motion of the Sun • The Sun’s path (as viewed from Earth) defines the plane of the ecliptic. • The Moon, planets and most asteroids all lie close to the plane of the ecliptic b/c the solar system is nearly flat (like a pancake). • The Sun’s declination changes during the year depending on where it is in its path--this is the cause of seasons.
The ecliptic is tilted 23.5º from the celestial equator. NCP VE SS WS AE SCP Tilt of Ecliptic
Sun’s path in the sky • The sun’s apparent path in the sky (rising in the east and setting in the west) is “caused” by Earth’s rotation • During one day, the Sun only moves 1/365 of the way around the ecliptic. • Therefore, it moves very little in one day. • The location along the horizon where the Sun rises and sets depends on where the Sun is located on the ecliptic.
Location of the Sun’s path east west The actual path depends on time of year (i.e. where the Sun is located on the ecliptic). Sometimes it rises higher, and sometimes it rises lower in the sky.
Question • When is the Sun directly overhead (at the zenith) at noon in High Point? • Every day of the year • At the vernal equinox • At the spring equinox • At the summer solstice • At the winter solstice • It will never appear directly overhead as viewed in High Point • Answer: F
NCP VE SS WS AE SCP Seasons • Seasons are determined by the location of the Sun on the ecliptic. • Near the summer solstice, the Sun has a greater declination above the equator. • At greater declinations, our latitude experiences the most direct sunlight (summer).
Question • The average temperature (at a certain location on the Earth) when the Sun is at the autumnal equinox is ________ the average temperature at the same location when the Sun is at the vernal equinox. • greater than • less than • equal to • Answer: equal to
Practice • A friend asks, “Why is it hot in the summer and cold in the Winter?” Write an answer to your friend. • Your grandma asks “Why is it summer in the United States when it is winter in Australia? Write an answer to your grandma. • Compare your response to someone else’s response in the class.
The Moon • The Moon orbits in 29.5 Earth days (one lunar month) • The Moon exhibits a cycle of phases as the illuminated surface seen from earth changes • The Moon’s orbital plane is tilted 5 degrees to the ecliptic.
Phases of the Moon Cycle of Phases -- one month animation day 1 - new day 3.5 waxing crescent day 7 first quarter day 10.5 waxing gibbous day 14 - full day 17.5 waning gibbous day 21 last quarter day 24.5 waning crescent
Eclipses • The plane of the Moon’s orbit is tilted 5º from the ecliptic, so eclipses are rare. • Lunar eclipse--when the sun is between the Earth and Moon at a time when the moon would normally be full. As a result, the Earth shades the moon and it “disappears.” • Solar eclipse--when the Moon is between the Earth and Sun (new moon) and its shadow is cast on the Earth.
Question • Does everyone in a certain hemisphere see a solar or lunar eclipse at the same time or at least on the same day (assume it’s daytime or nighttime in all of those locations)? • yes • no • for solar eclipses, yes, but for lunar eclipses, no. • for lunar eclipses, yes, but for solar eclipses no. • It depends on the date the eclipse occurs, so sometimes yes and sometimes no. • Answer: B
Question • Can a total solar eclipse occur when the moon is not full or new? • yes • no • it depends... • Answer: B
Question • What is the dark side of the moon? • The side of the Moon that nobody on Earth can see. • The side of the Moon that we in High Point can’t see, although others on Earth can see it. • The side of the Moon that never faces the Sun. • none of the above • Answer: A
Question • In the movie, Apollo 13, the actors view the dark side of the moon and comment on the fact that they would have landed there. Why would this have been a bad idea? • It would have been too cold and dark. • They could have only communicated with the southern hemisphere. • They could have only communicated with the northern hemisphere. • They could not have communicated with anyone at all. • We would not be able to see them with our television cameras. • Answer: D
The night sky • “Groups of stars” are called constellations. • Constellations were officially defined by IAU in 1930. • There are 88 constellations. • These groupings appear together in the sky, although the stars may be very different distances.
Zodiac • Twelve constellations • Significant in astrology • Aligned with the plane of the ecliptic; thus, the Sun passes through these constellations. Leo Virgo Libra Scorpius Sagittarius Capricornus Aquarius Pices Aries Taurus Gemini Cancer
Zodiac Scorpius Libra Virgo Sagittarius Leo 3/21 Capricornus Cancer 6/21 12/21 9/21 Aquarius Gemini Pices Taurus Aries
Capricornus 6/21 Zodiac Cancer • In mid June the sun is “in” Cancer • Overhead at night is Capricorn
Summary • Our reference frame for observation is the Earth. Although, it’s in motion about the Sun, from our reference frame (i.e. perspective) it appears that everything else is in motion with respect to the Earth. • The Celestial Sphere provides a coordinate system and a useful model for understanding the perceived motion of the Sun, Moon, and Stars. • Using this model, we can explain the seasons and phases of the moon