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Lecture 3, ASTA01

Chapter 2User’s Guide to the Sky:

Patterns and Cycles

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Precession

• In addition to the daily motion of the sky, Earth’s rotation adds a second motion to the sky that can be detected only over centuries.

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Precession

• More than 2100 years ago, astronomer Hipparchus compared a few of his star positions with those made by other astronomers nearly two centuries before him.• He realized that the celestial poles and

equator were slowly moving relative to the stars. • Later astronomers understood that this

apparent motion is caused by a special motion of Earth called precession.

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Precession

• If you have ever played with a toy top or gyroscope, you may recall that the axis of such a rapidly spinning object sweeps around relatively slowly in a circle. • The weight of the top tends to

make it tip.• This combines with its rapid

rotation to make its axis sweep around slowly in precession motion.

Precession

• Earth spins like a giant top, but it does not spin upright relative to its orbit around the Sun.• You can say either that Earth’s axis is tipped

23.5°from vertical or that Earth’s equator is tipped 23.5° relative to its orbit.

• “obliquity” 23.5°

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Precession

• Earth’s large mass and rapid rotation keep its axis of rotation pointed toward a spot near Polaris (alpha Ursa Minoris).• Its axis direction would not move if Earth were

a perfect sphere.• However, Earth has a slight

bulge around its middle.• The gravity of the Sun and

Moon pull on this bulge, tending to twist Earth’s axis upright relative to its orbit.

Precession

• The combination of these forces and Earth’s rotation causes Earth’s axis to precess in a slow circular sweep – taking about 26 000 years for one cycle.

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Precession

• As the celestial poles and equator are defined by Earth’s rotational axis, precession moves these reference marks. • You would notice no change at all from night

to night or year to year.• Precise measurements, though, reveal their slow

apparent motion.

Precession

• Over centuries, precession has dramatic effects. • Egyptian records show that 4800 years ago

the north celestial pole was near Thuban (alpha Draconis). • Now, the pole is

approaching Polaris and will be closest to it in about 2100.

Precession

• In about 12,000 years, the celestial pole will have moved to the apparent vicinity of the very bright star Vega (alpha Lyrae).• The figure shows

the apparent path followed by the north celestial pole over thousands of years.

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The Cycle of the Sun

• Rotation is the turning of a body on its axis.

• Revolution is the motion of a body around a point outside the body. • Earth rotates on its axis – and that produces

day and night. • Earth also revolves around the Sun – and that

produces the yearly cycle.

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The Annual Motion of the Sun

• Why don’t we see stars during day??• Even in the daytime, the sky is actually

filled with stars.• However, the glare of sunlight fills Earth’s

atmosphere with scattered light, and you can only see the brilliant blue sky.

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The Annual Motion of the Sun

• If the Sun were fainter and you could see the stars in the daytime, you would notice that the Sun appears to be moving slowly eastward relative to the background of the distant stars.• This apparent motion is caused by the real

orbital motion of Earth around the Sun.

The Annual Motion of the Sun

• In January, you would see the Sun in front of the constellation

Sagittarius. By March, it is in front of Aquarius.

The Annual Motion of the Sun

• Note that your viewing angle in the figure makes the Earth’s orbit seem very elliptical when it is really almost a perfect circle.

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The Annual Motion of the Sun

• Through the year, the Sun moves eastward among the stars following a line called the ecliptic – the apparent path of the Sun among the stars.• If the sky were a great screen, the ecliptic

would be the shadow cast by Earth’s orbit.• In other words, you can call the ecliptic the

projection of Earth’s orbit on the celestial sphere.

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The Annual Motion of the Sun

• Earth circles the Sun in 365.26 days and, consequently, the Sun appears to go once around the sky in the same period.• You don’t notice this motion because you

cannot see the stars in the daytime.• However, the motion of the Sun caused by a real

motion of Earth has an important consequence that you do notice – the seasons.

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The Seasons

• The seasons are caused by the revolution of Earth around the Sun combined with a simple fact you have already encountered.• Earth’s equator is

tipped 23.5°relative to its orbit.

The Seasons

• There are two important principles to note about the cycle of seasons.

The Seasons

• The seasons are not caused by variation in the distance between Earth and the Sun.

• Earth’s orbit is nearly circular, so it is always about the same distance from the Sun. In fact, the sun is closer in the winter!

The Seasons

• The seasons are caused by changes in the amount of solar energy that Earth’s northern and southern hemispheres receive at different times of the year – resulting from the tip of the Earth’s equator and axis relative to its orbit.

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The Seasons

• The seasons are so important as a cycle of growth and harvest that cultures around the world have attached great significance to the ecliptic.• It marks the centre line of the zodiac (‘circle of

animals’).• Also, the motion of the Sun, Moon, and the five

visible planets (Mercury, Venus, Mars, Jupiter, and Saturn) are the basis of the ancient superstition of astrology.• However, the signs of the zodiac are no longer

important in astronomy.

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The Planets

• You can look for the planets along the ecliptic appearing like very bright stars.• Mars looks quite orange in colour. • Saturn and Jupiter are silver and usually brighter• Venus is the brightest. Sometimes it’s so close to

the sun (in degrees) that it’s hidden in sun’s glare. • Uranus and Neptune cannot be seen by naked

eye. Pluto & asteroids are hard to see even with moderate-sized telescopes.

The Planets

• As Venus and Mercuryorbit inside Earth’s orbit,they never get far from the Sun and are visible in the west after sunset or in the east before sunrise. • Venus can be very bright,

but Mercury is difficult tosee near the horizon.

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The Planets

• By tradition, any planet in the sunset sky is called an evening star.• Any planet in the dawn sky is called a

morning star. • Both names are misnomers! They’re not

“stars”.

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The Planets

• Venus, which can become as bright as magnitude -4.7

• That’s almost 100 times brighter than Vega• As Venus moves around its orbit, it can

dominate the western sky each evening for many weeks.

• Eventually, its orbit appears to carry it back toward the Sun as seen from Earth, and it is lost in the haze near the horizon.

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The Planets

• A few weeks later, you can see Venus reappear in the dawn sky as a brilliant morning star. Example: September 2012.• Months later, it will switch

back to being an evening star.

The Cycles of the Moon

• The Moon orbits eastwards around Earth once a month.

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The Cycles of the Moon

• Starting this evening, look for the Moon in the sky. • If it is a cloudy night or if the Moon is in the

wrong part of its orbit, you may not see it.• Keep trying on successive evenings.• Within a week or two, you will see the Moon. • Then, watch for the Moon on following

evenings.• You will see it move along its orbit around Earth

and cycling through its phases as it has done for billions of years.

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The Motion of the Moon

• If you watch the Moon night after night, you will notice two things about its motion. • First, you will see it moving relative to the

background of stars. A related thing is that the Moon rises and sets about 50 minutes later every day. Every day it’s seen to the east of the previous position.

• Second, you will notice that the markings on its face don’t change. • These two observations will help you understand the

motion of the Moon and the origin of the Moon's phases.

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The Motion of the Moon

• The Moon moves continuously & rapidly among the constellations. • If you watch the Moon for just an hour, you

can see it move eastward against the background of stars by slightly more than its own apparent diameter.• Each night when you look at the Moon, you will

see it is roughly half the width of a zodiac constellation—about 13 degrees—to the east of its location the night before. • This movement is the result of the motion of the Moon

along its orbit around Earth.

The Cycle of Moon Phases

• The changing shape of the illuminated part of the Moon as it orbits Earth is one of the most easily observed phenomena in astronomy.

The Cycle of Moon Phases

• There are three important points to notice about the phases of the Moon.

The Cycle of Moon Phases

• First, the Moon always keeps the same side facing Earth, and you never see the far side of the Moon. Spin and orbit are in 1:1 period ratio.

• ‘The man in the moon’ (some cultures see ‘the rabbit in the moon’ instead) is produced by familiar features on the Moon's near

side.

Q: Is there a “dark side of the Moon” ??

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The Cycle of Moon Phases

• Second, the changing shape of the Moon as it passes through its cycle of phases is produced by sunlight illuminating different parts of the side of the Moon you can see.

The Cycle of Moon Phases - day 2

Gray moon

The Cycle of Moon Phases – day 5

The Cycle of Moon Phases – day 7

The Cycle of Moon Phases – day 14.5

The Cycle of Moon Phases – siderial (27.3 days) & synodic (29.5 days) periods

The Cycle of Moon Phases – siderial (27.3 days) & synodic (29.5 days) periods

Earth

Siderial = with respect to starsSynodic = meeting other bodies in the same configuration

The Cycle of Moon Phases – day 27

Watch the Youtube video with accurate representation of moon phases during one full year: http://www.youtube.com/watch?v=7JqVqvIlrwA

http://www.youtube.com/watch?v=7JqVqvIlrwA

This is an animation of what you’d see over 12 months (in 2012) if you were able to watch the Moon all the time. It’s based on detailed maps of the Moon.

The Moon seems to wobble left and right – that’s a true physical effect called libration; it’s due to its orbit’s ellipticity and thus uneven motion of the Moon around Earth.

It also tilts up and down – it’s not a physical effect, rather the Moon is on an orbit slightly inclined to the Earth’s equator so over one month we do see it a bit from below (south pole better than north pole) and then from above (more of a north pole).