Discovering the Night Sky

An Astronomer's Timeline

• 586 B.C.: Thales of Miletus predicts a solar eclipse.

• ca. 270 B.C.: Aristarchus of Samos proposes a heliocentric cosmology.

• 350 B.C.: Aristotle proposes a spherical Earth and a geocentric cosmology.

• 2136 B.C.: Chinese astronomers record a solar eclipse.

• ca. A.D. 125: Claudius Ptolemy refines and details geocentric cosmology in his Almagest.

• 1512-1543: Nicolaus Copernicus proposes heliocentric cosmology in Commentariolus and De Revolutionibus Orbium Coelestium.

• 1576-1601: Tycho Brahe makes precise observations of stars and planets.

• 1589-1609: Galileo Galilei proposes that all objects fall with the same acceleration (independent of mass) and builds his first refracting telescope.

• 1609-1610: Johannes Kepler publishes his three laws of planetary motion.

• 1665-1704: Isaac Newton deduces gravitational force from the Moon's orbit, builds the first reflecting telescope, proves that planets moving under gravitational force obey Kepler's laws, and publishes Opticks (compendium on light).

• 1715: Edmond Halley calculates the shadow path of a solar eclipse over Earth's surface.

• 1766: Henry Cavendish discovers hydrogen.

• 1800-1803: William Herschel discovers infrared radiation from the Sun. Thomas Young demonstrates the wave nature of light. John Dalton proposes that matter is composed of atoms of different masses.

• 1840-1849: J.W. Draper invents astronomical photography, taking the first photographs of the Moon. Christian Doppler proposes that wavelength is affected by motion. Lord Rosse completes a 60-inch reflecting telescope. Armand Fizeau and Jean-Bernard Foucault measure the speed of light accurately.

• 1871-1873: Dimitri Mendeleyev develops the periodic table of elements. Henry Draper develops spectroscopy. James Clerk Maxwell asserts that light is an electromagnetic phenomenon.

• 1885-1888: Johann Balmer expresses spectral lines of hydrogen mathematically. Heinrich Hertz detects radio waves.

• 1895-1897: Wilhelm Roentgen discovers X-rays. Joseph Thomson detects the electron. The Yerkes 40-inch optical refracting telescope is completed.

• 1900: Max Planck explains blackbody radiation. Paul Villard discovers gamma rays.

• 1913: Niels Bohr proposes quantum theory of the atom.

• 1930-1934: Karl Jansky builds the first radio telescope. James Chadwick discovers the neutron. Bernhard Schmidt builds his Schmidt optical reflecting telescope.

• 1942-1949: J.S. Hey detects radio waves from the Sun. The first astronomical telescope is launched into space. Herbert Friedman detects X-rays from the Sun. The 200-inch optical reflecting telescope begins operation at Mt. Palomar, California.

• 1963-1967: The largest single-dish radio telescope (300 m across) begins operation at Arecibo, Puerto Rico. The first Very Long Baseline Interferometer (VLBI) images are produced.

• 1975: First Charge Coupled Device (CCD) astronomical observations.

• 1980: Very Large Array (VLA) radio observatory completed in Socorro, New Mexico.

• 1990-1996: The Hubble Space Telescope is launched. Keck 10-m optical/infrared telescopes begin operation at Mauna Kea, Hawaii. The SOHO solar observatory is launched.

• 1999: The Chandra X-ray Telescope is launched.

• 2004-present: Two rovers travel on Mars; detectors search for gravitational radiation.

Key Questions

• Is Polaris the brightest star? No.

• Do astronomers see constellations as star patterns? Sometimes, or it is the entire area of the celestial sphere.

• What causes the seasons? The tilt of the Earth's axis.

• When is the Earth closest to the Sun? Around January 3rd.

• How many zodiac constellations are there? 13.

• Does the Moon have a dark side? Half the moon.

• Is the Moon ever visible during the daytime? Yes.

• What causes lunar and solar eclipses? Alignment of the Sun, Earth, and Moon.

Chapter Overview

• How astronomers organize the night sky.

• Earth's rotation causes day and night.

• Earth's axial tilt and orbit cause seasons.

• Moon's orbit causes lunar phases and eclipses.

• How the year is defined and the calendar was developed.

Scientific Notation

• Used to describe very large and very small numbers.

• Involves powers of ten (10^n).

• Examples:

  • 10^3 = 1,000 (one thousand, Kilo)

  • 10^9 = 1,000,000,000 (one billion, Giga)

  • 10^{-6} = 0.000001 (one millionth, micro)

Scales of the Universe

• Study objects from subatomic particles to galaxies and the known universe.

• Each division indicates a 100,000 increase in size.

Astronomical Objects

• Planets (e.g., Jupiter)

• Stars (e.g., Sun)

• Black holes

• Rocky and metallic debris (asteroids)

• Rocky and icy debris (comets)

• Interstellar gas and dust (nebulae)

• Galaxies

• Intergalactic gas

• Clusters of galaxies

Constellations

• Regions of the sky named after familiar star patterns.

• Ancient constellations were imaginary pictures outlined by star patterns.

• Modern astronomers divide the sky into 88 official constellations.

Finding Constellations

• Using the "Big Dipper" as a guide

• Summer Triangle (Deneb, Vega, Altair)

• Winter Triangle

Celestial Sphere

• Imaginary sphere surrounding the Earth where objects in the sky are located.

• Celestial poles: points directly above Earth's poles.

• Celestial equator: divides the celestial sphere into northern and southern hemispheres.

• Positions defined by right ascension and declination.

Earth's Motions

• Rotation: Earth's spin on its axis (one day).

• Revolution: Earth's movement around the Sun (one year).

• Precession: Slow conical motion of Earth's axis (26,000 years).

Angular Measurements

• Angular distance between the pointer stars of the Big Dipper is about 5°.

• Use the human hand at arm's length to estimate angles.

Circumpolar Stars

• Stars near the celestial poles that circle the pole and never set.

Apparent Motion in the Sky

• Westward motion of Sun, Moon, and stars caused by Earth's rotation.

• Objects rise in the east and set in the west at middle latitudes.

Day and Night

• Side of Earth facing the Sun experiences day.

• Side turned away experiences night.

Star Motion at the Poles

• Stars move in horizontal circles due to Earth's rotation.

• At the North Pole, stars move left to right.

• At the South Pole, stars move right to left.

Star Motion at the Equator

• Stars rise straight up on the eastern horizon and set straight down on the western horizon.

Star Motion at Middle Latitudes

• Stars rise and set at an angle that changes throughout the night.

• Latitude determines the angle of rising and setting.

Earth's Revolution and the Ecliptic

• Sun appears to move through the stars along the ecliptic.

• Ecliptic plane: the plane of Earth's orbit.

Seasons

• Caused by the tilt of the Earth's axis.

• Different parts of Earth receive more direct sunlight (summer) or more spread out sunlight (winter).

Solstices and Equinoxes

• Summer solstice (June 21): Sun's path farthest north.

• Winter solstice (December 21): Sun's path farthest south.

• Vernal equinox (March 21): Sun moving north.

• Autumnal equinox (September 21): Sun moving south.

• Seasons are opposite in the Southern Hemisphere.

Sun's Daily Path

• Winter solstice: Sun rises farthest south of east, lowest in the sky, shortest time above horizon, least intense light.

• Vernal equinox: Sun rises precisely in the east, sets in the west.

• Summer solstice: Sun rises farthest north of east, highest in the sky, longest time above horizon, most intense light.

• Autumnal equinox: Same conditions as vernal equinox.

Midnight Sun

• Sun is above the horizon continuously at high latitudes (e.g., 69° N) from mid-May until the end of July.

Time Zones

• Used to calculate the time of day in any part of the world.

Precession

• Gravitational forces of the Sun and Moon cause Earth to undergo a top-like motion.

• Earth's rotation axis moves in a circular motion over 26,000 years.

• Causes the position of the North Celestial Pole to change over time.

• Today, Polaris is the "North Star"; in 3000 BC, Thuban was, and in 14,000 AD, Vega will be.

Lunar Cycle

• Phases of the Moon are due to changing amounts of the lit side facing Earth.

• Synodic period (phase cycle) is 29½ days.

Sidereal vs. Synodic Month

• Synodic month: time for Moon to orbit Earth with respect to the Sun (29½ days).

• Sidereal month: time for Moon to orbit Earth with respect to the stars (27.3 days).

• Times differ because Earth moves in its orbit around the Sun.

Daytime Moon

• The Moon is visible during the day.

Eclipses

• Occur when the Moon, Sun, and Earth are aligned.

• Moon may enter Earth's shadow, or Moon's shadow may reach Earth.

• Do not occur every full or new moon because the Moon's orbit is tilted by 5°.

Types of Lunar Eclipses

• Penumbral: Moon appears dimmed.

• Partial: Part of Moon enters Earth's umbra and is darkened.

• Total: All of Moon enters Earth's shadow and becomes reddish.

Solar Eclipses

• Total solar eclipse: Moon covers the entire disk of the Sun, revealing the solar corona.

• Occur at specific places on Earth.

Eclipse Paths

• The Moon’s shadow travels along the eclipse path.

• Total Solar and Annular Eclipses occur between 2001 and 2020.

Annular Eclipses

• Occur when the Moon is too far from Earth to completely cover the Sun.

• A thin ring or annulus of light surrounds the Moon.

Sizes in Astronomy

• Objects range from ~10^{-15} m (parts of an atom) to ~10^{26} m (observable universe).

• Scientific notation is used to write very large and small numbers.

Patterns of Stars

• Celestial sphere divided into 88 constellations.

• Boundaries run along lines of constant right ascension or declination.

Earthly Cycles

• The celestial sphere appears to revolve around the Earth once in each day-night cycle, caused by the Earth’s rotation

• The poles and equator of the celestial sphere are determined by extending the axis of rotation and the equatorial plane of the Earth out onto the celestial sphere.

Earth's Axial Tilt

• Earth’s axis of rotation is tilted at an angle of 23½° from the perpendicular to the plane of the Earth’s orbit (the plane of the ecliptic).

• This tilt causes the seasons.

Key Points

• Equinoxes and solstices are significant points along the Earth’s orbit that are determined by the relationship between the Sun’s path on the celestial sphere (the ecliptic) and the celestial equator.

• The Earth’s axis of rotation slowly changes direction relative to the stars over thousands of years (precession).

• Precession is caused by the gravitational pull of the Sun and Moon on the Earth’s equatorial bulge.

• The length of the day is based upon the Earth’s rotation rate and the average motion of the Earth around the Sun, producing the 24 hour day.

Lunar Phases

• Phases of the Moon are caused by the relative positions of the Earth, Moon, and Sun.

• The Moon completes one cycle of phases in a synodic month, which averages 29½ days.

• The Moon completes one orbit around the Earth with respect to the stars in a sidereal month, which averages 27.3 days.

Shadows

• The shadow of an object has two parts: the umbra, where direct light from the source is completely blocked; and the penumbra, where the light source is only partially obscured.

Lunar Eclipses

• A lunar eclipse occurs when the Moon moves through the Earth’s shadow.

• During a lunar eclipse, the Sun, Earth, and Moon are in alignment with the Earth between the Sun and the Moon, and the Moon is in the plane of the ecliptic.

Solar Eclipses

• A solar eclipse occurs when a strip of the Earth passes through the Moon’s shadow.

• During a solar eclipse, the Sun, Earth, and Moon are in alignment with the Moon between the Earth and the Sun, and the Moon is in the plane of the ecliptic.

• Depending on the relative positions of the Sun, Moon, and Earth, lunar eclipses may be penumbral, partial, or total, and solar eclipses may be annular, partial, or total.

Key Terms

• angle.

• angular diameter (angular size).

• annular eclipse.

• arc angle.

• celestial equator.

• celestial pole.

• celestial sphere.

• circumpolar star.

• constellation.

• declination.

• degree.

• diurnal motion.

• eclipse path.

• ecliptic.

• equinox.

• gravitation.

• line of nodes.

• lunar eclipse.

• lunar phase.

• north celestial pole.

• partial eclipse.

• penumbra.

• penumbral eclipse.

• precession.

• precession of the equinoxes.

• revolution.

• right ascension.

• rotation.

• scientific notation.

• sidereal month.

• sidereal period.

• solar corona.

• solar day.

• solar eclipse.

• south celestial pole.

• summer solstice.

• synodic month.

• terminator.

• time zone.

• total eclipse.

• umbra.

• vernal equinox.

• winter solstice.

• zenith.

• zodiac

What Did You Think?

• Is the North Star—Polaris—the brightest star in the night sky?

  • No. Polaris is a star of medium brightness compared with other stars visible to the naked eye.

• Do astronomers regard constellations as the familiar patterns of stars in the sky?

  • Astronomers sometimes use the common definition of a constellation as a pattern of stars. Formally, however, a constellation is an entire area of the celestial sphere and all the stars and other objects in it. Viewed from Earth, the entire sky is covered by 88 different-sized constellations. If there is any room for confusion, astronomers refer to the patterns as asterisms.

• What causes the seasons?

  • The tilt of the Earth’s rotation axis with respect to the ecliptic causes the seasons. They are not caused by the changing distance from the Earth to the Sun that results from the shape of Earth’s orbit.

• When is the Earth closest to the Sun?

  • On or around January 3 of each year.

• How many zodiac constellations are there?

  • There are 13 zodiac constellations, the lesser-known one being Ophiuchus.

• Does the Moon have a dark side that we never see from Earth?

  • Half of the Moon is always dark. Whenever we see less than a full Moon, we are seeing part of the Moon’s dark side. So, the dark side of the Moon is not the same as the far side of the Moon, which we never see from Earth.