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Unit 11 Astronomy
Chapter 30 What is Astronomy?
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30.1 Cycles on Earth
 A calendar is a means of keeping track of all the days in a year.
 The development of a calendar came from a need to be able to predict the seasons, annual
floods, and other cyclical occurrences in communities’ lives.
 The day and the year, as units of time, are based upon the motion of the earth.
 Modern calendars also add one day every four years to account for the fraction of a day
travelled by Earth around the sun with a leap year.
 Earth’s rotation (around its axis) causes day and night, Earth’s revolution (around the sun) lead
to the passage of years.
 rotation is the spinning of Earth on its axis
 Day and night occur on Earth because of Earth’s rotation around its axis
 revolution is the movement of one object around another in space
 Earth completes one revolution of its orbit around the sun in 365.25 days.
 Many calendars of today also indicate moon movement, although the moon’s movement
doesn’t affect which day of the month or year it is.
 One month closely coincides (although not exactly) with the lunar cycle
 The lunar cycle is when the moon makes one complete revolution around the Earth
 The moon moves around Earth in a path called an orbit.
 An orbit is a regular, repeating path that an object in space follows around another object.
 How the moon appears to Earth dwellers at different positions in its orbit is shown below:
 The phases of the moon occur because of the positions of Earth, the moon, and the sun.
Unit 11 Astronomy
Chapter 30 What is Astronomy?
 Earth’s axis is the imaginary line that passes through its center and connects the North and
South poles.
 Earth’s axis is tilted 23.5° with respect to the plane waves of the sun’s light, and the North Pole
of the Earth always points to Polaris, the North Star.
 One year (365.25 days) is the amount of time it takes Earth to complete one revolution around
the sun.
 What causes the seasons?
 Because Earth’s axial tilt remains fixed as it revolves around the sun, different parts of
Earth receive more direct sunlight at different times during the year.
Unit 11 Astronomy
Chapter 30 What is Astronomy?
 In the Northern Hemisphere, the sun is as low in the sky (farthest south) as it ever is
during the year on December 21st, it is as high in the sky (farthest north) as it ever is on
June 21st.
 See illustration below.
 Movement of the Earth and its moon can also cause eclipses.
 During a solar eclipse, the moon is directly between the Earth and the sun. The moon’s
shadow falls on Earth.
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Solar eclipses occur at (or close to) new moon phase
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Total solar eclipses are rare because the moon casts such a small shadow
compared to the size of the Earth.
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Total solar eclipses are observed directly under the shadow of the moon and
last for about three minutes, partial solar eclipses are observed in the area
around the total solar eclipse.
 During a lunar eclipse, the Earth is directly between the sun and the moon. The Earth’s
shadow falls on the moon.
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Lunar eclipses occur at (or close to) full moon phase
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The moon’s orbit is tilted at a 5° angle with respect to Earth’s orbit around the
sun.
Unit 11 Astronomy
Chapter 30 What is Astronomy?
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Because of this, lunar eclipses do not occur every month.

Eclipses can be total or partial, depending on how much of the moon is covered
by Earth’s shadow.
30.2 Tools of Astronomy
 The closest star to our sun, Alpha Centauri, is 41,000,000,000,000 kilometers away.
 Trying to write out astronomical distances requires a lot of zeros.
 Scientific notation is a mathematical abbreviation for writing very large (or very small) numbers.
The steps for changing numbers into scientific notation are:
1. Move the decimal until you get a value that is between 1 and 10. Count the number
of times you move the decimal.
Unit 11 Astronomy
Chapter 30 What is Astronomy?
2. Write down the new number without all of the zeros.
3. Write “× 10” after the number
4. Write the number of times you moved the decimal as the power of 10 (the
exponent). If you moved the decimal to the left, the exponent will be positive. If
you moved the decimal to the right, the exponent will be negative.
 The universe is defined as everything that exists, including all matter and energy.
 A galaxy is a huge collection of gas, dust, and billions of stars.
 A star is a sphere of gas that undergoes a process called fusion. This process releases so much
energy, the star gives off very bright light.
 A planet is a large, spherical piece of matter that revolves around a star.
 The brightest object in the night sky (besides the moon) is the planet Venus.
 Venus does not give off its own light (no planets do) but instead reflects light from the
sun, just like the moon does
 A light year is the distance light travels in one year through space (9.46 × 1012 kilometers).
 The light year is a unit of distance (not length!!) and is useful in measuring the distance
from Earth to the stars
 Example: How many light years away is Alpha Centauri, the closest star to our sun? Remember
that this star is 4.1 trillion km away…..4.1 × 1013 km = ? light years
 Example: How many seconds does it take for light to travel to Earth from the sun, which is 150
million kilometers away? Light travels at a speed of 300,000,000 km/sec.
Unit 11 Astronomy
Chapter 30 What is Astronomy?
 A telescope is any device that collects radiation, which may be in the form of electromagnetic or
particle radiation, from a limited direction in space.
 Electromagnetic radiation is radiation consisting of electromagnetic waves, including
radio waves, infrared, visible light, ultraviolet, x-rays, and gamma rays. Light is one part
of the spectrum of electromagnetic radiation.
 Telescopes can be refracting, reflecting, radio, IR, or X-ray
 A refracting telescope is a telescope in which a lens gathers light and forms a real image
of an object (they are also known as refractor telescopes). In refracting telescopes, a
lens gathers the light from distance objects, and then another lens in the eyepiece
magnifies the image.
 A reflecting telescope is a telescope that uses mirrors instead of lenses to gather and
focus light.
 Radio telescopes receive EM radio waves that cannot be seen but can travel great
distances. The signals are received and a computer creates an image of the source of
the radio waves. Radio telescopes are used to produce images of stars and galaxies,
analyze the chemical composition of objects, and map the surfaces of planets.
Unit 11 Astronomy
Chapter 30 What is Astronomy?
 X-ray telescopes are designed to detect high-energy radiation (X-rays) from space. Xrays from space cannot penetrate our atmosphere, so X-ray telescopes must be placed
on an object that leaves Earth’s atmosphere (like a satellite).
 An observatory is an observing site on Earth that contains a large telescope. Observatories are
usually located on hilltops, away from densely populated areas. The world’s largest refracting
telescope is housed at Yerkes Observatory in Williams Bay, Wisconsin.
 A satellite is an object that travels in orbit around another object.
 The Hubble Space Telescope (or HST) is located on a satellite that orbits Earth. This powerful
telescope collects images of deep space and sends information back to computers on Earth.
 Space probes are unmanned spacecraft that carry scientific instruments on board. These probes
fly past planets and send information back to Earth. They are not designed to return to Earth.
 The International Space Station is a joint project of six nations that orbits 450 km above Earth’s
surface. On board, scientists conduct numerous experiments, many of which depend on
constant freefall (microgravity) conditions provided by the space station.
 Space shuttles are piloted spacecraft that launch from rocket “boosters” and can land back on
Earth like an airplane.
 A space shuttle consists of three big components: two solid rocket boosters (SRBs), an external
fuel tank, and an orbiter.
Unit 11 Astronomy
Chapter 30 What is Astronomy?