Astronomy

Ancient AstronomyAstronomy probably began long before recorded history (more than 5,000 years ago) when human began to track the motion of celestial objects so they knew when to plant their crops or prepare to hunt migrating herd.The ancient Chinese, Egyptians, and Babylonians are well known for their record keeping.These cultures recorded the location of the Sun, Moon, and the five visible planets as these objects moved slowly against the background of “fixed” stars.It was not enough to track the motions of celestial objects; predicting their future positions (to avoid getting married at an unfavorable time, for example) become important.

A study of Chinese archives shows that the Chinese recorded every appearance of the famous Halley’s Comet for at least 10 centuries.Like most ancients, the Chinese considered comets to be mystical, generally comet were seen as bad omen and were blamed for a variety of disaster, from wars to plague.

The Chinese has an accurate records of “guest stars”. Today we know that a “guest star” is a normal star, usually too faint to be visible, which increases its brightness as it explosively ejects gases from its surface, a phenomena we call a Nova or Supernova.

The Golden Age of AstronomyThe Golden Age of early astronomy

(600 B.C. –AD 150) was centered in Greece.

The basics of Geometry and Trigonometry, which they developed, were used to measure the size of and distances to the largest-appearing bodies in the heavens –Sun and the Moon.

The Golden Age of early astronomy (600 B.C. –AD 150) was centered in Greece.

The early Greeks held the incorrect Geocentric (geo = Earth, centric = centered) view of the universe.Orbiting the

Earth were the moon, Sun, and known planets – Mercury, Venus, Mars, Jupiter and Saturn

Beyond the planets was a transparent, hallow celestial sphere on which stars were attached and travelled around the Earth.

The sun and the moon were thought to be perfect crystal sphere

The famous Greek Philosopher Aristotle (384-322BC) concluded that Earth is spherical because it always casts a curved shadow when it eclipses the moon.

Although most of Aristotle’s teaching were considered infallible by may for centuries after his death, his belief in a spherical Earth was lost during the Middle Ages.

Measuring the Earth’s Circumference

The first successful attempt to establish the size of the Earth was credited to Eratosthenes (276-194 B.C.)

Eratosthenes observed the angles of the noonday Sun in two Egyptian cities that were roughly north and south of each other- Syene (now Aswan) and Alexandria

Finding the angles of the noonday sun differed by 7 degrees, or 1/50 of a complete circle, he concluded that the circumference of Earth must be 50 times the distance between these two cities.The cities were 5,000 stadia apart, giving him a measurement of 250,000 stadia. Many historians believe the stadia was 157.6 meters, which would make Eratosthenes’s calculation of Earth’s circumference – 39,400 km.- a measurement very close to the modern value of 40. 0705 km.

The Sun-Centered Universe

The first Greek to profess a Sun-Centered, or Heliocentric , (helios=sun, centric=centered) universe was Aristarchus (312-230 B.C.)

Because of the strong influence of Aristotle’s writing, the Earth-centered view dominated Western thought for nearly 2,000 years.

Mapping the StarsThe greatest among Greek astronomers was Hipparchus, best known for his star catalogue.

Hipparchus determined the location of almost 850 stars, which he divided into six groups according to their brightness (the system is still used today)

He measured the length of the year to within minutes of the modern value and developed a method for predicting the times of lunar eclipse to within a few hours.

Many of the Greek discoveries were lost during the Middle Ages, the Earth-centered view that the Greek proposed became entrenched in Europe.

Claudius PtolemyMuch of our knowledge Of Greek

astronomy comes from a 13-volume treatise, Almagest (the great work) which was compiled by Ptolemy in A.D. 141.With the decline of the Roman Empire around fourth century, much of the accumulated knowledge disappeared as libraries were destroyed

After the decline of Greek and Roman civilization, the center of astronomical study move east to Baghdad, where fortunately, Ptolemy’s work was translated into Arabic

Arabic astronomers expanded Hipparchus’s star model catalog and divided the sky into 48 constellations – the foundation of our present-day constellation system.

It wasn’t until some time after the tenth century that the ancient Greeks’ contributions o astronomy were reintroduced to Europe through the Arabic community.

The Ptolemaic model soon dominated European thought as the correct representation of the heavens, which created problems for anyone who found errors in it.

The Birth of Modern AstronomyPtolemy’s Earth-centered universe was not

discarded overnight.

Modern astronomy’s development was more than scientific endeavor, it require a break from deeply entrenched philosophical and religious views that had been a basic part of Western society for thousands of years.

The work of five noted scientists undergo a transition from an astronomy that merely describe what is observed, to an astronomy that tries to explain what is observed and more importantly why the universe behaves the way it does.

Nicolaus CopernicusFor almost 13 centuries after the time of Ptolemy, very few astronomical advances were made in Europe, some were even lost, including the notion of a spherical Earth.The first great astronomer to emerge after

the Middle Ages was Nicolaus Copernicus (1473-1543) from PolandAfter discovering Aristarchus’s writing, Copernicus became convinced that Earth is a planet, just the other five then-known planet.The daily motions of the heavens, he reasoned could be more simply explained by rotating Earth.

Having concluded that the Earth is a planet, Copernicus constructed the heliocentric model for the solar system with the Sun at the center and the planets Mercury, Venus, Earth, Mars, Jupiter, and Saturn orbiting it.This was a major break from the ancient and

prevailing idea that a motionless Earth lies at the center of all movement in the universe.At that this time it was considered heretical b many Europeans. Anyone who refuse to denounce the Copernican theory were burned at the stake.

Tycho Brahe

Tycho Brahe was born of Danish nobility three years after the death of Copernicus.

He persuaded King Frederick II to establish an observatory near Copenhagen, which headed.There he designed and built pointers

(telescope would not be invented for a few more decades), which he used for 20 years to systematically measure the location of the heavenly bodies in an effort to disprove the Copernican theory

His observations, particularly of Mars, were far more precise than any made previously and are his legacy to astronomy.

With the death of his patron, the King of Denmark, Tycho was forced to leave his observatory.

Tycho moved to Prague in the present day Czech Republic, where, in the last year of his life, he acquired an able assistant, Johannes Kepler.Kepler retained most of the observations made by Tycho and put them to exceptional use.

Ironically, the data Tycho collected to refute the Copernican view of the solar system would later be used by Kepler to support it.

Johannes KeplerArmed with Tycho’s data, a good

mathematical mind, and, of greater importance, a strong belief in the accuracy of Tycho’s work, Kepler derived three basic laws of planetary motion1. The path of each planet around the Sun, while almost circular, is actually an ellipse, with the sun at one focus.

This law allow us to calculate astronomical events like eclipses, comets, spacecraft rendezvous, and satellite action.

2. Each planet revolves so that an imaginary line connecting it to the Sun sweeps over equal areas in equal intervals of time. In order for a

planet to sweep equal areas in the same amount of time, it must travel more rapidly when it is nearer the Sun (perihelion) and more slowly when it is farther from the sun (aphelion).

Kepler was devout and believed that the Creator made an orderly universe and that this order would be reflected in the positions and motions of the planets.

The uniformity he tried to find eluded him for nearly a decade.

Then in 1619, Kepler published his third law in the Harmony of the Worlds

3. The planet orbital period squared is equal to its mean solar distance cubed.

The solar distances of the planets can be calculated when their periods of revolutions are known.

For example: Mars has an orbital period of 1.88 years, which squared equals 3.54. The cube root of 3.54 is 1.52, and that is the average distance from Mars to the Sun in astronomical unit.

Kepler’s law assert that the planets revolve around the Sun, and therefore support the Copernican theory.

Galileo GalileiHe was the greatest Italian

scientists of the Renaissance.

He was a contemporary of Kepler, and like Kepler, strongly supported the Copernican theory of a Sun-centered solar system.

Galileo’s greatest contributions to science were his descriptions of the behavior of moving objects, which he derived from experimentation.

All astronomical discoveries before Galileo’s time were made without the aid of a telescope.

In 1609, Galileo heard that a Dutch lens maker had devised a system of lenses that magnified objects.

Apparently without ever seeing the telescope, Galileo constructed his own, which magnified distant object three times the size seen by the unaided eye.

He immediately made others, the best having magnification of about 30.

With the telescope, Galileo was able to view the universe in a new way.

Galileo made many discoveries that supported the Copernican view of the Universe including the following:1. The discovery of Jupiter’s four largest satellite or moons.

This find dispelled the old idea that Earth was the sole center of motion in the universe; for here, plainly visible, was another center of motion –Jupiter.

2. The discovery that the planets are circular disk rather than just points of light, as was previously thought.

This indicated that the planets must be Earth-like revolved around the sun.

3. The discovery that Venus exhibits phases just as the Moon does and that Venus appears smallest when it is in full phase and thus is farthest from Earth

This observation demonstrates that Venus orbits its source of light –the Sun.

In the Ptolemaic system, the orbit of Venus lies between Earth and the Sun, which means that only the crescent phases of Venus should ever be seen from Earth.

4. The discovery that Moon’s surface is not a smooth glass sphere, as the ancient had proclaimed.

Galileo saw mountains, craters, and plains indicating that the Moon was Earth-like.

He thought that plains might be bodies of water, and this idea was strongly promoted by others, as we tell from the names given to these features (sea of tranquility, sea of storms, etc.

5. The discovery that the sun (the viewing of which may have caused the eye damage that later blinded him) has sunspot – dark regions caused by slightly lower temperatures.Galileo tracked the

movement of these spots and estimated that rotational period of the Sun as just under a month. Hence, another heavenly body was found to have both “blemishes” and rotational motion.

In 1616, the Church condemned the Copernican Theory as contrary to Scripture because it did not put humans as their rightful place at the center of Creation, and Galileo was told to abandon this theory.Undeterred, Galileo began writing his famous work, Dialogue of the Great World Systems.

Despite poor health, he completed the project and in 1630 went to Rome, seeking permission from Pope Urban VIII to publish.Since the book was a dialogue that expounded both the Ptolemaic and Copernican system, publication was allowed.However, Galileo’s detractors were quick to realize that he was promoting Copernican view at the expense of the Ptolemaic system.

The sale of the book was quickly halted, and Galileo was called before the Inquisition.

Tried and convicted of proclaiming doctrines contrary to religious teaching, he was sentenced to permanently house arrest, under which he remained for the last 10 years of his life.Despite this restriction, and his grief following the death of his eldest daughter, Galileo continued to work.In 1637 he became totally blind, yet during the next few years he completed his finest scientific work, a book on the study of motion in which he stated that the natural tendency of an object in motion is to remain in motion.Later, as more scientific evidence in support of the Copernican system was discovered, the Church allowed Galileo’s works to be published.

Sir Isaac NewtonSir Isaac Newton (1642-1727) was born in the year of Galileo’s death.

His many accomplishments in Mathematics and Physics led a successor to say that “Newton was the greatest genius that ever existed.”

At the age of 23, he envisioned a force that extends from Earth into space and holds the Moon in orbit around Earth.

He was the first to formulate and test the Law of Universal Gravitation. It states that:

“Every body in the universe attracts every other body with a force that is directly proportional to their masses and inversely proportional to the square of the distance between them”

The Law of gravitation also states that the greater the mass of an object, the greater its gravitational force.

ConstellationsThe division of the sky into areas.

Constellation (con = with, stella = star)

Usually named in honor of mythological character or great heroes.

Sometimes it takes a bit of imaginations to make out the intended subjects, as most constellation were probably not thought of as likeness in the first place.

Summer Constellation in

the Northern Hemisphere

The Big Dipper

Although the stars that make up constellation all appear to be the same distance from Earth, this is not the case.

Some are many times farther away than others.

Thus, stars in a particular constellation are not associated with each other in any important physical way.

Various cultural groups, including Native Americans and the Chinese, attached their names, pictures, and stories to the constellations.

For example, the constellation Orion the hunter was known as the White Tiger to ancient Chinese observer.

Astronomers use constellations when they want to roughly identify the area of the heaven they are observing.

Some of the brightness stars in the heavens were given proper names such as Sirius, Arcturus, and Betelgeuse.

The brightness stars in a constellation are generally names in order of their brightness by the letter of the Greek alphabet –alpha (α), beta (β), and so on – followed by the name of the parent constellation.

For example, Sirius, the brightness star in the constellation Canis Major (larger Dog) is also called Alpha (α) Canis Majoris.

Motion of Earth

1. Rotation

The main consequences of Earth’s rotation are day and night.

Earth’s rotation has become a standard method of measuring time because it is so dependable and easy to use.

Earth’s rotation is measured in two ways, making two kinds of days.1) Mean Solar Day

The time interval from one noon to the next, which averages about 24 hours.Noon is when the Sun has reached its zenith (highest point in the sky).

2) Sidereal (sider=star, at=pertaining to) dayIs the time it takes for Earth to make one complete rotation (3600) with respect to a star other than our sun.The sidereal day is measured by the time required for a star to reappear at the identical position in the sky.The sidereal day has a period of 23 hours, 56 minutes, and four seconds, which is almost four minutes shorter than the mean solar day.This difference results because the directions to distant stars changes only infinitesimally, whereas the direction to the Sun changes by almost 1 degree each day.

2. Revolution

Earth revolves around the Sun in an elliptical orbit at an average speed of 107, 000 km per hour.

Its average distance from the sun is 150 million km.

Because of its elliptical orbit. Earth’s distance from the sun varies.

Earth’s axis is tilted about 23.50.

This angle is very important to Earth’s inhabitants because the inclination of Earth’s axis causes the yearly cycle of seasons.

3. Precession

A very slow movement of the Earth is called axial precession.

Although Earth’s axis maintains approximately the same angle of tilt, the direction in which the axis point continually changes. As a result, the axis traces a circle in the sky.

At present time, the axis points toward the bright star Polaris. In AD 14,000, it will point toward the bright star Vega, which will then be the North Star for about a thousand years or so. The period of precession is 26,000 years.By the year 28,000, Polaris will once again be the North Star.

Precession has only a minor effect on the season because Earth’s angle of tilt changes slightly.

Motion of the Earth-Moon System

Earth has one natural satellite, the Moon.

In addition to accompanying Earth in its annual trek around the Sun, our Moons orbits Earth about once each month.

When viewed from a Northern Hemisphere perspective, the Moon moves counterclockwise (eastward) around the earth.

The Moon’s orbit is elliptical, causing the Earth-Moon distance to vary by about 6 percent, averaging 384,401 km.

Lunar MotionThe cycle of the Moon through its phases requires 29 ½ days – a time span called the synodic month.

This cycle was the basis for the first Roman calendar

Sidereal Month is the apparent period of the Moon’s revolution around the Earth and not the true period, which takes only 27 1/3 days.

As the moon orbits Earth, the Earth-Moon system also moves in an orbit around the Sun. Consequently,

even after the Moon has made a complete revolution around Earth, it has not yet reached its starting position with respect to the Sun, which is directly between the Sun and the Earth.

An interesting fact concerning the motions of the Moon is that its period of rotation around its axis and its revolution around the Earth are the same -27 1/3 days.Because of this, the same lunar hemisphere always faces Earth.

All of the landings of the manned Apollo missions were confined to the Earth-facing side.Only orbiting satellite and astronaut have seen the back side of the Moon.

Because of the Moon rotates on its axis only once every 27 1/3 days, any location on its surface experiences periods of daylight and darkness lasting about two weeks.Along with the absence of atmosphere, accounts for the high surface temperature of 1270C (2610F) on the day side of the moon and the low surface temperature of -1730C (-2800F) on its night side.

Phases of the MoonThe first astronomical phenomenon to be understood was the regular cycle of the Phases of the Moon.

On a monthly basis, we observe the phases as a systematic change in the amount of the Moon that appear illuminated.

Phases of the Moon

We will choose the “new-Moon” position in the cycle as the starting point.

About 2 days after the new Moon, a then silver (crescent phase) can be seen with the naked eye low in the western sky just after sunset.

During the following week, the illuminated portion of the moon that is visible from the Earth increases (waxing) to a half-circle (first-quarter phase) that can be seen from about noon to midnight.

In another week, the complete disk (full-Moon phase) can be seen rising in the east as the Sun sinks in the west.

During the next two weeks, the percentage of the Moon that can be seen steadily declines (waning), until the Moon disappears altogether (new-Moon phase).

The lunar phases are a consequence of the motion of the Moon and the sunlight that is reflected from its surface.

Half of the Moon that is illuminated at all times. But on the Earthbound observer, the percentage of the bright side that is visible depends on the location of the Moon with respect to the Sun and Earth.

When the moon lies between the Sun and Earth, none of its side faces Earth, so we see the new-Moon (“no moon”) phase.

When the moon lies on the side of Earth opposite the Sun, all of its lighted side faces Earth, so we see the full Moon.

Eclipses of the Sun and MoonWhen the Moon moves in a line directly between Earth and the Sun, which can occur only during the new-Moon phase, it casts a dark shadow on Earth, producing a solar eclipse (eclipsis=failure to appear)

The Moon eclipse (lunar eclipse) when it moves within Earth’s shadow, a situation that is possible only during the full-Moon phase.

The Moon’s orbit is inclined about 50 to the plane of the ecliptic. Thus,

a) during the most new-Moon phases, the shadow of the Moon passes either above or below Earth.b) During most full-Moon phases, the shadow of Earth misses the Moon.

An eclipse can only take place when a new- or full Moon phase occurs while the Moon’s orbit crosses the place of the ecliptic.

Crossing the plane of ecliptic are normally met twice a year, therefore the usual number of eclipses is four.

These occur a set of one solar and one lunar eclipse, followed six months later with another set.Occasionally the alignment is such that three eclipses can occur in a one month period –at the beginning, middle, and the end.These occur as a solar eclipse flanked by two lunar eclipse, or vice versa.

It also occasionally happens that the first set of eclipses for the year occurs at the very beginning of a year, the second set in the middle, and the third set occurs before the calendar year ends, resulting in six eclipses in the year.More rarely, if one of these sets consists of three eclipses, the total number of eclipse is a year can reach seven, which is the maximum.

Total eclipses are visible only to people in the dark part of the Moon’s shadow (umbra), while a partial eclipse is seen by those in the light portion (penumbra).

SAQ’s

1. Why do we use the mean solar day to measure time rather than the sidereal day?

2. Why did the ancients believed that celestial objects had some control over their lives?

3. What major change did Copernicus make in the Ptolemaic system? Why was this change philosophically significant.

4. What was Tycho Brahe’s contribution to science?

5. Does Earth move faster in its orbit near perihelion (January) or near aphelion (July)? Keeping your answer to previous question in mind, is the solar day longest in January or July?

6. Use Kepler’s third law (p2 = d3 ) to determine the period of a planet whose solar distance is:

a) 10 AUb) 1 AUc) 0.2 AU

7. Use Kepler’s third law to determine the distance from the sun of a planet whose period is

a) 5 yearsb) 10 yearsc) 10 days

8. Did Galileo invent the telescope?

9.Of what value are constellations to modern-day astronomers?

10. Explain the difference between the mean solar day and the sidereal day.

11. What is the different about the crescent phase that precedes the new-Moon phase and that which follows the new-Moon phase?

12. What phases of the Moon occurs approximately one week after the new-Moon? Two weeks?

13. Currently, Earth is closest to the sun (perihelion) in January (147 million km) and farthest from the sun in July (152 million km). As the result of the precession of Earth’s axis, 12,000 years from now perihelion ( closest) will occur in July and aphelion (farthest) will take place in January. Assuming no other changes, how might this change average summer temperature for your location? What about average winter temperature? What might the impact on the biosphere and hydrosphere?

14. In what ways do the interactions between Earth and its Moon influence the Earth system? If Earth did not have a Moon, how might the atmosphere, hydrosphere, geosphere, and biosphere be different?

15. Describe the locations of the Sun, Moon, and Earth during a solar eclipse and during a lunar eclipse.