Motions in the Heavens

Earth rotates on its axis and simultaneously revolves or orbits around the Sun. Which of these two motions is responsible for each of the following:

SunEarth

Earth

The sun rises in the east and sets in the west

Is this because the Earth rotates on its axis, or because the earth orbits the sun?

The sun rises in the east and sets in the west

Is this because the Earth rotates on its axis, or because the earth orbits the sun?

The earth rotates on its axis from east to west.

Different constellations appear in the summer than in the winter

Is this because the Earth rotates on its axis, or because the earth orbits the sun?

+10 +10

SunEarth Earth

−10 −10Ophiuchus Orion

Observed region of night sky during one season

Observed region of night sky six months later

Different constellations appear in the summer than in the winter because of the Earth’s orbit around the sun.

Orion’s BeltThe ancient Greeks saw this…….and imagined this!

At night, stars in the Northern Hemisphere appear to revolve around the North Star

Is this because the Earth rotates on its axis, or because the earth orbits the sun?

At night, stars in the Northern Hemisphere appear to revolve around the North Star

Is this because the Earth rotates on its axis, or because the earth orbits the sun?The stars seem to rotate because the Earth rotates on its axis.

Moon revolves around Earth while Earth rotates on its axis

Which of these two motions is responsible for the following?

The moon rises in the east and sets in the west, just like the sun.

Is this because the earth rotates on its axis or because the moon orbits around the earth?

The moon rises in the east and sets in the west, just like the sun

And like the sun, it is because the earth rotates on its axis.

The moon rises approximately 53 minutes later each day.

Is this because the earth rotates on its axis or because the moon orbits around the earth?

The moon rises approximately 53 minutes later each day.

The moon rises once a day because the earth rotates on its axis.

Its 53 minutes later each day, because the moon orbits the earth and therefore moved “forward” by a small amount.

Why the moon rises later each night

• Full moon rises at about sunset.

• In 24 hours, the earth makes a complete rotation, but the moon has move ahead on its orbit, so the earth must rotate “a bit extra” for the moon to rise above the horizon. This extra bit is about 53 minutes.

The moon passes through monthly phases. Is this because the earth rotates on its axis or because the moon orbits the earth?

The moon passes through monthly phases.

The moon passes through monthly phases. Is this because the earth rotates on its axis or because the moon orbits the earth?

The phase of the moon depends where it is in its orbit around the earth.

Why are there phases of the moon?

• The moon only reflects light from the Sun.

• When the moon is at point E (farthest from sun), entire lit face can be seen.

• At point A, only dark side points towards Earth.

• B-D, F-H, occur in between full moon and new moon.

Planets• Move independently of

stars.• Sometimes appear to

move “backwards” relative to other heavenly bodies (“retrograde motion”).

• Larger and brighter than most stars.

• Don’t twinkle because they are much closer than stars.

Aristotle and the Geocentric (Earth-centered) Universe

• All heavenly bodies orbited the Earth, including the Sun: – People have no sensation of motion so it must

be the sky that moved– People do not fall off the Earth, so it must not

be moving (at least not suddenly)– Stars position relative to each other never

changes • Modified by Ptolemy to explain retrograde

motion of planets.

Aristotle’s Geocentric Universe

The Renaissance and the Heliocentric Solar System

The Beginning: Copernicus (1473-1543)– Polish astronomer found Ptolemaic system too

complicated– Proposed a heliocentric (sun-centered) model that was

more simple than Ptolemy’s adaptation of Aristotles’s model

– Not censored by the Church at the time, but also not widely circulated.

Brahe & Kepler• Brahe (1546-1601)

mapped all known objects in the sky, but did not explain their motions.

• Kepler (1561-1630) Brahe’s assistant, who believed in the heliocentric theory, proposed the idea of elliptical orbits for planets.

Galileo (1564-1642) – the Father of Modern Science

– the Scientific Method: the laws of nature must be understood by observation, experimentation, and analysis.

– Developed a telescope far better than those of previous astronomers

– Used his observations of heavenly motions and analysis of earthly motion to question the geocentric model

Galileo – Study of Physics

Law of Inertia – the tendency for an object to resist a change in motion. People living on a planet moving at a constant speed would realize no motion, unless it were suddenly accelerated.

Galileo – Study of Milky Way and Moon with telescope contradicts Aristotle’s Ideas

• Saw Milky Way was a collection of individual stars, not a cloud of light.

• Realized the Moon was not an unblemished body.

Fig. 22.13a, p.567

Final Proof for Galileo of the Heliotropic Solar System

• Moons of Jupiter orbited Jupiter, not the Sun.

• Phases of Venus not possible for in a geocentric configuration.

Galileo was persecuted by the Roman Catholic Church for his

beliefs

Isaac Newton (1642-1727)

Why do planets orbit around the Sun, instead of flying off into space?

Newton's work combined the contributions of Copernicus, Kepler, Galileo, and others to answer this question by explaining the unversal laws of gravity and motion

22.4 The motions of the Earth and the Moon

• By 1700, the heliocentric system was well established– rotation – the Sun and planets spin on their

axes– Revolution – the Earth traveling around the

sun in its orbit– Precession – the wobble of Earth’s axis

Fig. 22-15, p.535

Fig. 22-16, p.536

The Moon

• Revolves around Earth in 29.5 days• Rotates on its axis in 29.5 days

– New moon – when the moon is dark– Crescent moon – four days later– Gibbous moon – bright moon 10 days after– Full moon – 14-15 days after the new moon– Waning moon – as it shrinks toward the next

new moon

Orbital plane of earth and moon• The Moon’s plane of

orbit is tilted 5.2o

relative to that of the Earth

• Thus the Earth’s shadow misses the full moon.

• The new moon’s shadow usually misses the Earth.

Eclipses of the Sun and Moon

• If the Moon passes through the Earth–Sun plane when the three bodies are aligned properly, then an eclipse will occur

Eclipses of the Sun and Moon

• Solar eclipse – when the Moon passes directly between Sun and Earth– Umbra – the region of

shadow exhibiting total eclipse

– Penumbra – the edge of shadow showing partial eclipse

• Lunar eclipse – When the Earth lies in line between the Moon and the Sun.– Last longer and more

common as Earth has a bigger shadow

Fig. 22-22, p.540

Total Solar Eclipse

Partial Solar Eclipse

Fig. 22-23, p.540