Lecture 2 : Visual Astronomy Stars and Planets Robert Fisher

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Lecture 2 : Visual Astronomy -- Stars and Planets

  • Robert Fisher


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    • 5 PM Tuesday
    • 5 PM Thursday
    • 12 Noon Friday
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Review of Lecture 1

  • Astronomy is an ancient subject, passed down from Greek to Islamic scholars, and transmitted back to the west.

  • Our systems of thought evolve with time at an almost imperceptibly slow pace, and continue to do so today.

  • The universe is thought to have begun with a big bang, and is expanding.

  • The cosmic calendar varies over fantastically-long timescales. We are very recent newcomers onto the cosmic scene.

  • We are all stardust.

Overview of Lecture 2

  • I. The Celestial Sphere

  • II. The Stars

  • IiI. The Motion of the Planets

Important Lessons to be Learned

  • Because the stars are very distant, their motion on the sky is well-described as if they revolved around the Earth

  • The motion of the planets is significantly more complex, and required elaborate geometrical constructions in the ancient geocentric system due to Ptolemy

  • Johannes Kepler’s three laws of planetary motion captures most features of planetary motion extremely well in a heliocentric model

Motion of the Stars

  • The foundation of all visual astronomy is a simple fact : the Earth is a Sphere

  • While common knowledge today, determination of the shape of the Earth was a significant challenge to ancient peoples

  • The most convincing elementary argument comes from the fact that the Earth’s shadow (as seen in lunar eclipses) is always circular, as Aristotle correctly deduced

Celestial Sphere, Zenith, Nadir, Horizon

Zenith and Nadir Depend on Your Location

Motion of the Celestial Sphere

The Motion of the Sun

  • At a given location, the sun rises towards the east and sets towards the west.

  • A sundial gnomon casts a shadow away from the sun, towards the west.

  • The invention of the gnomon is attributed to the ancient Greek philosopher Animaxander, successor to Thales

Determining North from the Sun’s Motion

  • At noon, the sun reaches its highest point in the sky, directly north.

  • This was a common method used by the ancients to determine North.


  • In the afternoon, the sun begins to set in the west, following the same circular arc traced in the morning.

  • The direction traced by the sun in its arc, facing north, is clockwise.

Great Circle


  • Separation between two points on the celestial sphere are measured in terms of angle.

  • A full circle is 360 degrees.

  • Each degree is 60 minutes.

    • The full moon is roughly one-half degree in width.
    • By remarkable circumstance, the width of the sun is also one-half degree.
  • Each minute is 60 seconds -- sometimes referred to as arcseconds.

The Meridian

  • The great circle on the celestial sphere found by connecting north and south and passing through the zenith is referred to as the meridian.

  • When a celestial body crosses the meridian, it is said to transit.

  • When a body transits, it reaches its highest point from the horizon.

  • The terms “AM” and “PM” derive their meaning from the meridian :

    • AM = Ante-Meridian
    • PM = Post-Meridian

The North Celestial Pole and Circumpolar Stars

  • Looking north from Chicago at night, one can see the North Celestial Pole.

  • The North Celestial Pole is the direction along which the Earth’s axis is aligned.

  • The stars which immediately surround the pole never set beneath the horizon. They are called circumpolar stars.

Star Trails Over Mauna Kea, Hawaii

Daily Motion of the Stars

  • The daily motion of the stars Is very simple.

  • The celestial sphere makes one full circle about the Earth, once per day.

  • The circle is determined by only angle -- the declination.


  • In the Northern hemisphere, the stars rise in the East, set in the West, and revolve counter-clockwise around the North celestial pole. In the southern hemisphere the stars rise in the

    • A) East, set in the West, and revolve anti-clockwise around the South celestial pole.
    • B) East, set in the West, and revolve clockwise around the South celestial pole.
    • C) West, set in the East, and revolve clockwise around the South celestial pole.
    • D) West, set in the East, and revolve anti-clockwise around the South celestial pole.

View from North Pole

  • At the north pole, the zenith is the north celestial pole.

  • The nadir is the south celestial pole.

  • The horizon is the celestial equator.

  • Precisely half of the celestial sphere is visible.

  • All stars are circumpolar.

View from Equator

  • The zenith is the celestial equator.

  • The north celestial pole always appears directly north.

  • The full sky is visible -- each star rises for 12 hours each day.

View from Chicago

  • The altitude of the north celestial pole is equal to the latitude of your position on the Earth - roughly 42 degrees for Chicago.

  • Stars within 42 degrees of the north celestial pole are circumpolar.

  • Stars within 42 degrees of the south celestial pole are not visible.

Summary of Celestial Sphere Viewed fom Earth


  • The celestial equator is :

    • A) The path of the sun compared with the stars.
    • B) The path of the moon compared with the stars.
    • C) The average path of planets on the sky.
    • D) Always directly overhead at the Earth’s equator.
    • E) Always along the horizon at the Earth’s equator.


The Ecliptic

The Solstices and Equinoxes

  • The solstices occur when the sun reaches a maximum (solstice = sol sistere or sun stops in Latin) in declination -- roughly June 21 and December 21.

  • The equinoxes occur when the sun intersects the celestial equator -- roughly March 21 and September 21. On this day, the sun appears directly above the equator, and every point on earth has equal day and night.

Earth on Equinoxes

Yearly Sky and Zodiac

Angle of Inclination of Earth

  • The ecliptic makes an angle of 23.5 degrees with the celestial equator.

  • Physically, this means the Earth’s rotational axis is tilted with respect to its orbit.

Angle of Inclination

  • As the Earth orbits around the sun, the angle of inclination remains the same.

Origin of Seasons

  • The angle of inclination causes seasonal variation on Earth.


  • The ecliptic makes its smallest angle with the southern horizon during the

    • A) Summer
    • B) Autumn
    • C) Winter
    • D) Spring

Lunar Phases

  • The appearance of the moon varies over the course of the month.


  • The lunar orbit is inclined by 5 degrees relative to that of the Earth/sun.

  • Solar eclipses can occur during the new moon, but only when the sun, moon, and Earth happen to line up.

  • Similarly, lunar eclipses can occur during the full moon, but only when the sun, Earth, and moon happen to line up.

Lunar Eclipses

  • The moon passes through the shadow of the Earth.

  • Light is fully blocked in the umbra, and only partially blocked in the penumbra.

Types of Lunar Eclipses

  • Three types of Lunar eclipses.

Lunar Eclipses

Lunar Eclipses from Moon

Solar Eclipses

  • Solar eclipses occur when the sun’s light is blocked by the moon.

  • In a sense, they are completely serendipitous : the sun is 400 times larger than the moon, but is also 400 times further away.

  • Hence, the apparent angular size of both the moon and the sun are nearly identical.

Solar Eclipses

  • Three types of solar eclispes can occur.

August 11, 1999 Eclipse Viewed from Mir

Solar Eclipses, 1999 - 2020

The Planets

The Motion of Planets

  • Like the stars, the planets are generally seen to traverse the sky.

  • Unlike the stars, occasionally the planets are observed to stop and move from west-to-east in so-called retrograde motion.

  • This behavior gave rise to the ancient greek name -- “planets” comes from a Greek root meaning “wanderer”.

  • A fully satisfactory explanation of this motion was not developed until Newton.

Retrograde Motion

Ptolemaic Model of the Solar System

  • The ancient astronomer Ptolemy (90 - 168 AD) created the most complex version of the geocentric model of the system, which was used for almost one and a half millenia.

  • In the Ptolemaic model, the moon, sun, and planets all revolved in circles, which themselves revolved around circles around the Earth.

  • And in fact, the Earth was not quite at the center of this model, either.

Why Did the Ancients Reject a Heliocentric Model of the Solar System?

  • In the heliocentric model, due to the motion of the Earth about the sun, the motion of the nearest stars should appear to vary with respect to the more distant stars.

  • This effect is called parallax.

  • The ancients attempted to measure this effect, but failed. In fact, because the stars are so distant, it is only detectable with telescopic measurements.

Phases of Venus

  • In 1610, Galileo used the telescope to observe the phases of Venus for the first time from the Earth.

  • The phases only made sense if Venus orbited the Sun, not the Earth.

  • This proved to be a “smoking gun” in favor of the heliocentric model.

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