Science 3210 001 : Introduction to Astronomy

Lecture 2 : Visual Astronomy -- Stars andPlanets

Robert Fisher

Items

❑ Add/Drop Day

❑ Office Hours Vote

❑ 5 PM Tuesday❑ 5 PM Thursday❑ 12 Noon Friday

❑ Course Webpage

❑ Questions

Review of Lecture 1

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

❑ Our systems of thought evolve with time at an almostimperceptibly slow pace, and continue to do so today.

❑ The universe is thought to have begun with a big bang, and isexpanding.

❑ 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, andrequired elaborate geometrical constructions in the ancientgeocentric system due to Ptolemy

❑ Johannes Kepler’s three laws of planetary motion captures mostfeatures of planetary motion extremely well in a heliocentricmodel

Motion of the Stars

❑ The foundation of all visualastronomy is a simple fact :the Earth is a Sphere

❑ While common knowledgetoday, determination of theshape of the Earth was asignificant challenge toancient peoples

❑ The most convincingelementary argument comesfrom the fact that the Earth’sshadow (as seen in lunareclipses) is always circular,as Aristotle correctly deduced

Earth Image, Apollo 17 Crew

Celestial Sphere, Zenith, Nadir, Horizon

❑ The distant stars appear to lie on a solid sphere, the celestial sphere.

❑ The zenith is the direction directly upwards.

❑ The nadir is the direction directly downwards.

❑ The horizon splits the celestial sphere in half along the zenith-nadir axis.

Zenith and Nadir Depend on Your Location

❑ The zenith and nadir directions depend on where one stands on theEarth.

Motion of the Celestial Sphere

❑ The rotation of the Earth causes the celestial sphere to appear torevolve.

❑ The north/south celestial poles correspond to the north/south poles of theEarth’s rotational axis.

The Motion of the Sun

❑ At a given location, the sun rises towards the east and sets towards thewest.

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

❑ The invention of the gnomon is attributed to the ancient Greekphilosopher 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.

Clockwise

❑ In the afternoon, the sun begins to set in the west, following the samecircular arc traced in the morning.

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

Great Circle

A great circle on a sphere divides the sphere into two hemispheres.

One can imagine the equator as an example of a great circle, but any circle dividing the sphere is a great circle.

Great circle

Angles

❑ Separation between two points on the celestial sphere aremeasured 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-halfdegree.

❑ Each minute is 60 seconds -- sometimes referred to asarcseconds.

The Meridian

❑ The great circle on the celestial sphere found by connecting north and south andpassing 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 CelestialPole.

❑ The North Celestial Pole is the direction along which the Earth’s axis isaligned.

❑ The stars which immediately surround the pole never set beneath thehorizon. 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.

Question

❑ In the Northern hemisphere, the stars rise in the East, set in theWest, and revolve counter-clockwise around the North celestialpole. In the southern hemisphere the stars rise in the

❑ A) East, set in the West, and revolve anti-clockwise around the Southcelestial pole.

❑ B) East, set in the West, and revolve clockwise around the Southcelestial pole.

❑ C) West, set in the East, and revolve clockwise around the Southcelestial pole.

❑ D) West, set in the East, and revolve anti-clockwise around theSouth celestial pole.

View from North Pole

❑ At the north pole, the zenith isthe north celestial pole.

❑ The nadir is the south celestialpole.

❑ The horizon is the celestialequator.

❑ Precisely half of the celestialsphere is visible.

❑ All stars are circumpolar.

View from Equator

❑ The zenith is the celestialequator.

❑ The north celestial pole alwaysappears directly north.

❑ The full sky is visible -- eachstar rises for 12 hours each day.

View from Chicago

❑ The altitude of the north celestialpole is equal to the latitude ofyour position on the Earth -roughly 42 degrees for Chicago.

❑ Stars within 42 degrees of thenorth celestial pole arecircumpolar.

❑ Stars within 42 degrees of thesouth celestial pole are notvisible.

Summary of Celestial Sphere Viewed fom Earth

Question

❑ 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.

Constellations

Constellations are the “states” on maps of the celestial sphere.

Each region of the sky belongs to precisely one constellation.

Stars within each region are alphabetically named, starting with thebrightest stars, by a greek letter followed by the constellation name -- eg,Polaris is Alpha Ursae Minoris.

The Ecliptic

The sun appears to move along a plane in the sky referred to as theecliptic.

The other planets also appear to move close to the ecliptic.

Physically, the fact that all solar system bodies lie close to the ecliptic isbecause everything lies within a flattened disk.

The Solstices and Equinoxes

❑ The solstices occur when the sun reaches a maximum (solstice = solsistere or sun stops in Latin) in declination -- roughly June 21 andDecember 21.

❑ The equinoxes occur when the sun intersects the celestial equator --roughly March 21 and September 21. On this day, the sun appearsdirectly above the equator, and every point on earth has equal day andnight.

Earth on Equinoxes

Yearly Sky and Zodiac

As the sun moves through the ecliptic, different portions of the night skybecome observable.

The ecliptic falls into 12 constellations over the year -- the 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 toits orbit.

Angle of Inclination

❑ As the Earth orbits around the sun, the angle of inclination remains thesame.

Origin of Seasons

❑ The angle of inclination causes seasonal variation on Earth.

Question

❑ The ecliptic makes its smallest angle with the southern horizonduring the

❑ A) Summer

❑ B) Autumn

❑ C) Winter

❑ D) Spring

Lunar Phases

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

Eclipses

❑ 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 whenthe 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 thepenumbra.

Types of Lunar Eclipses

❑ Three types of Lunar eclipses.

Lunar Eclipses

Lunar Eclipses from Moon

Solar Eclipses

❑ Solar eclipses occur when thesun’s light is blocked by themoon.

❑ In a sense, they are completelyserendipitous : the sun is 400times larger than the moon, butis also 400 times further away.

❑ Hence, the apparent angularsize of both the moon and thesun are nearly identical.

Solar Eclipses

❑ Three types of solar eclispes canoccur.

August 11, 1999 Eclipse Viewed from Mir

Solar Eclipses, 1999 - 2020

Both lunar and solar eclipses recur with a frequency of 18 years, 11 days,known as the Saros cycle.

The Saros cycle was known to the ancient Babylonians, and wasprobably used by Thales to predict the eclipse of May 18, 584 BC.

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 stopand 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 developeduntil Newton.

Retrograde Motion

❑ The mystery of retrogrademotion can be explainedrelatively simply in a heliocentricmodel of the solar system.

❑ An inner bod y (like the Earth) ismoving more rapidly than anouter body (like Mars), and sowill “pass” it like a faster car onthe expressway.

❑ During this passing, the outerplanet will execute retrogrademotion.

Ptolemaic Model of the Solar System

❑ The ancient astronomer Ptolemy(90 - 168 AD) created the mostcomplex version of thegeocentric model of the system,which was used for almost oneand a half millenia.

❑ In the Ptolemaic model, themoon, sun, and planets allrevolved in circles, whichthemselves revolved aroundcircles around the Earth.

❑ And in fact, the Earth was notquite at the center of this model,either.

Why Did the Ancients Reject a HeliocentricModel of the Solar System?

❑ In the heliocentric model, due tothe motion of the Earth about thesun, the motion of the neareststars should appear to vary withrespect to the more distant stars.

❑ This effect is called parallax.

❑ The ancients attempted tomeasure this effect, but failed. Infact, because the stars are sodistant, it is only detectable withtelescopic measurements.

Phases of Venus

❑ In 1610, Galileo used thetelescope to observe the phasesof Venus for the first time fromthe Earth.

❑ The phases only made sense ifVenus orbited the Sun, not theEarth.

❑ This proved to be a “smokinggun” in favor of the heliocentricmodel.