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CHAPTER 8: OUR PLACE IN SPACE
ASTRONOMY:

ASTRONOMY is the branch of science that studies objects beyond Earth.

Any object in space (for example: the Sun or the Moon) are considered to be CELESTIAL
OBJECTS.

Everything that physically exists is part of what we call the UNIVERSE. The universe includes
all energy, matter, and space.
STARS:

A STAR is a massive collection of gases, held together by its own gravity and emitting huge
amounts of energy.

Stars appear tiny in the sky because they are so far away.

We are able to see stars in the night sky because they are luminous.

LUMINOUS means that an object can produce its own light (it shines).
THE SUN:

The Sun is a star.

Compared to other stars, the Sun is average in size. However, it has a mass that is almost 340
000 times (or 3.4 x 105) that of Earth and a volume that is 1 300 000 times (or 1.3 x 106) the
volume of Earth!

The Sun appears to be so much bigger and brighter than other stars in the sky because of its
proximity to Earth: it is only 1.5 x 108 km away. The next closest star is nearly 300 000 times
farther away than the Sun! That’s 4.3 x 1013 km away!

Life would not be possible on Earth without the energy produced by the Sun. The Sun gives off
visible light and other forms of radiant energy, as well as solar wind (a stream of high-energy
particles).

Only a small fraction of the Sun’s light reaches Earth, but it is enough to keep water in its liquid
state and provide life on Earth with the energy needed for survival.

Stars and other celestial objects in the Universe emit energy consisting of electromagnetic waves
that travel at the speed of light, known as ELECTROMAGNETIC (EM) RADIATION.
Together, these forms of radiation are contained in the electromagnetic spectrum.

The ELECTROMAGNETIC (EM) SPECTRUM is the range of wavelengths of electromagnetic
radiation, extending from radio waves to gamma rays, and including visible light. These waves have
energies that become greater as their wavelengths become smaller.
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
The Sun emits radiation across most of the EM spectrum. Although some of the Sun’s energy is
absorbed by Earth’s atmosphere and some is reflected into space, almost all of the energy that
reaches Earth’s surface comes from the Sun.

EM radiation from the Sun is the driving force behind Earth’s weather and climate, and also
provides the energy needed for life to exist on Earth.

The Sun is composed of many layers of gas. Deep inside the Sun’s centre is the core, where high
temperatures and pressures cause particles to collide with each other at extremely high speeds.
This causes the particles to fuse, or join together, in a process called nuclear fusion. Nuclear
fusion gives off enormous amounts of energy. These high-energy reactions make the core of the
Sun the hottest part – reaching a temperature of 15 000 000 °C!

The energy release by nuclear fusion makes it way to the radiative zone (the first layer that
surrounds the core).

This energy can take up to a million years to reach the next region of the Sun – the convective
zone. It is in the convective zone that hotter substances rise as colder substances fall.

Energy continues to move outward until it reaches the photosphere, where light and other types
of radiation escape. The photosphere has a temperature of 5500 °C.

Above the photosphere lies the Sun’s atmosphere. It is divided into two layers: The
chromosphere and the corona. The chromosphere makes up the inner atmosphere and is 60 000
°C hotter than the photosphere. The corona is the thin outer layer of the Sun – it is gleaming
white, halo-like light, and extends millions of kilometres into space.
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
SUNSPOTS are dark spots appearing on the Sun’s surface that are cooler than the area
surrounding them. They are caused by disturbances in the Sun’s magnetic field.

SOLAR FLARES are gases and charged particles expelled above an active sunspot. They are
produced by the rapidly changing magnetic fields around sunspots and only last a short time.

SOLAR PROMINENCES are slow, low-energy ejections of gas that travel through the corona.
They still extend thousands of kilometres into space.
PLANETS:

A PLANET is a large celestial object that travels around a star.

There are 8 planets travelling around the Sun – Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune.
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
The four planets closest to the Sun are known as terrestrial planets. They have a hard and
rocky surface similar to Earth’s.

The next four planets are composed mostly of gases and liquids. They are known as gas giants.

Planets are non-luminous.

Even though planets do not produce and emit light, we can see planets because they reflect light
from luminous objects, such as the Sun and other stars.
EARTH:

Earth is the 3rd planet from the Sun, and the 4th largest planet in the solar system.

The SOLAR SYSTEM is the Sun and all the objects that travel around it.

Earth is a terrestrial planet composed primarily of rock.

Earth is in constant motion.

Earth differs from other planets in the solar system because it has a diversity of life forms and
large quantities of water.
MOONS:

A moon is a SATELLITE which is a celestial object that travels around a planet or a dwarf
planet.

The closed path of a celestial object or satellite as it travels around another (usually larger)
celestial object is called an ORBIT.

Mercury and Venus do not have moons in orbit; however, Jupiter and Saturn each have 60 or more
moons.

Earth has one natural satellite, called the Moon.
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o
The Moon is non-luminous – we are able to see it only because sunlight reflects off its
surface.
o
Although the Moon appears to be the biggest and brightest celestial object in the night
sky, it is small compared to the planets.
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It has a diameter four times smaller than that of Earth.
o
It appears large because it is close to Earth – it is 384 000 km away from Earth (about 55
times the distance between Vancouver, British Columbia, and St. John’s Newfoundland).
GALAXIES:

A GALAXY is a huge, rotating collection of gas, dust, and billions of stars, planets, and other
celestial objects.

Our Galaxy is the Milky Way. It contains more than 200 billion stars, including the Sun. It also
contains many other celestial objects.

The Milky Way appears as a hazy band of white light in the night sky.
The Sun’s Effects on Earth
THE AURORAS

Earth is surrounded by an atmosphere containing atoms of different gases, such as oxygen,
nitrogen, argon, and carbon dioxide. Earth is also surrounded by a magnetic field that is
strongest near the North and South Poles.

Solar winds travelling toward the Earth become influenced by, and follow, the lines of magnetic
force created by Earth’s magnetic field. Near the poles, they come in contact with particles in
Earth’s atmosphere, producing a display of light in the night sky.

In the northern hemisphere, we call these colourful displays of light the AURORA BOREALIS
or the northern lights.

In the southern hemisphere, we call them the AURORA AUSTRALIS or the southern lights.
They are simultaneously displayed when the northern lights are displayed.
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COMMUNICATION DISRUPTIONS

Solar activity at the Sun’s surface can affect artificial satellites (human-made objects that orbit
celestial objects in the Solar system).

For example, particles ejected from the Sun can damage the information stored on computer
microchips on satellites. This can disrupt cellphone and satellite TV communications.
RADIATION HAZARDS

On July 14, 2000, a powerful solar storm occurred. It is referred to as The Bastille Day Event.
Charged solar particles entered Earth’s atmosphere and disrupted signals from communication
satellites orbiting the planet. Solar scientists warned that people travelling in airplanes could
receive a higher than usual dose of radiation because of their high altitude.

The radiation in solar storms can also be harmful to astronauts during a space walk or through the
walls of their spacecraft while they orbit Earth because they are not protected by Earth’s
atmosphere.
The Solar System: The Sun and the Planets

The Solar System consists of the Sun, the eight planets and their moons, and billions of other
smaller celestial objects. All of these celestial objects orbit the Sun.

Our Solar System was once known as a geocentric model, meaning that everything revolves around
the Earth. However, Nicholas Copernicus (1473-1543) proposed a model placing the Sun at the
centre of the Solar System, called the heliocentric model of the Solar System. In the 1600’s,
Galileo confirmed this to be true by using a telescope to observe the planets.
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
Some planets are relatively close to the Sun. Mercury is just 58 million km away. However,
Neptune is almost 4 billion km from the Sun! It would take a spacecraft travelling at 28 000
km/h almost 50 years to cross the Solar System!

Because the distances in the Solar System are so large, astronomers use ASTRONOMICAL
UNITS or AU which is the average distance between the Sun and Earth. It is approximately
150 000 000 km (or 1.5 x 108 km). The AU is a more manageable way to measure astronomical
distances.

For example, the planet Jupiter is 780 million km from the Sun. This is equal to 5.2 AU – which is
more than five times farther from the Sun than Earth! [780 000 000 divided by 150 000 000].

For example, the planet Mercy is 0.387 AU from the Sun. This is equal to 58 million km [0.387 x
150 000 000].

The next largest objects in the Solar System (after the Sun) are the planets. The four planets
nearest the Sun are Mercury, Venus, Earth, and Mars. These small, rocky planets are considered
part of the inner Solar System. The four planets beyond Mars are Jupiter, Saturn, Uranus, and
Neptune. These planets lie in the outer Solar System and are known as the gas giant planets. As
their name implies, they are all big.

To be considered a planet; a celestial object must be in orbit around a star (such as the Sun),
have enough mass to be pulled into a stable sphere shape by gravity; and dominate its orbit.

A DWARF PLANET is a celestial object that orbits the Sun and has a spherical shape but does
not dominate its orbit. Currently, there are 5 recognized dwarf planets: Ceres, Pluto, Haumea,
Makemake, and Eris.

A METEOROID is a piece of metal or rock in the Solar System that is smaller than an asteroid.
Most meteoroids are the size of dust particles, but some can be as large as a car or building.
Sometimes, meteoroids can get pulled in by Earth’s gravity. As they are pulled down into Earth’s
atmosphere, friction causes them to burn up, creating a bright streak of light across the sky,
known as a METEOR. This phenomenon is commonly referred to as a “shooting star”. On rare
occasions, larger meteors do not burn up completely in the atmosphere and their remains, which
are called METEORITES, crash to the ground. An example of a meteorite crashing to Earth is
the Sudbury Basin, located in northern Ontario. It was formed by a 10 km meteorite that hit
Earth 1.85 billion years ago. It measures 62 km long by 30 km wide. Much of the nickel mined in
the Sudbury area originated from this meteorite.

A COMET is a large chunk of ice, dust, and rock that orbit the Sun. They range in size from less
than 100 m to more than 40 km across. Comets are classified as either short or long-period
comets. Short-period comets originate from a region just beyond the orbit of Neptune and
travel around the Sun in less than 200 years. For example, Halley’s Comet is the most famous
short-period comet, and takes 75 to 76 years to make on trip around the Sun. The last time
Halley’s Comet was visible was in 1986. It is expected to be seen again in 2061. Long-period
comets originate from a spherical cloud of debris much farther away than Pluto and take more
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than 200 years to orbit the Sun. For example, Comet Hale-Bopp takes 2380 years for it to make
a trip around the Sun.
Motions of Earth, the Moon, and Planets

The Sun always appears to rise from the East and set in the West. However, the Sun doesn’t
actually move across the sky, it only appears to do so.

The apparent motion of the Sun in the sky is caused by the rotation of Earth on its axis. Earth
makes one complete rotation, in a west-to-east direction, once each day. Earth’s rotations create
our day and night, and it takes Earth 24 hours to rotate once on its axis.

The Earth’s rotational axis is tilted at 23.5° from the vertical. This tilt affects the average
daytime temperature experienced by Earth’s hemispheres.

While Earth spins on its axis, it also revolves around the Sun. Earth’s orbit is elliptical – the
distance of Earth to the Sun changes as it completes its orbit – this is because the Sun is located
closer to one end of the elliptical path. Earth’s revolutions are what create our seasons, and it
takes Earth 365 days (one year) to complete one revolution around the Sun.

As Earth revolves around the Sun, the northern and southern hemispheres experience the
seasons – spring, summer, autumn, and winter. The Earth’s tilt is what is primarily responsible for
creating our seasons, not the distance that Earth is from the Sun. When Earth is farthest from
the Sun, the northern hemisphere is tilted toward the Sun and sunlight spreads over a relatively
small area of Earth’s surface. This causes intense heating of Earth’s surface and atmosphere.
During this time, the Sun appears to travel its highest path in the sky, and there are more hours
of daylight. When Earth is closest to the Sun, the northern hemisphere is tilted away from the
Sun and sunlight spreads over a larger area of Earth’s surface. This causes less heating of
Earth’s surface and atmosphere. During this time, the Sun appears to travel a lower path in the
sky and there are fewer daylight hours.

A SOLSTICE occurs twice a year. It is when the Earth’s axis is most inclined toward or away
from the Sun. Around June 21, Earth’s northern hemisphere is tilted toward the Sun as much as
possible. This is the longest day of the year, and is considered to be the first day of summer in
the northern hemisphere. Around December 21, the northern hemisphere is tilted away from the
Sun as much as possible. This is the longest night of the year, and is considered to be the first
day of winter in the northern hemisphere. When the northern hemisphere is tilted toward the
Sun, the southern hemisphere is tilted away – so the southern hemisphere experiences winter
when we experience summer.
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
Between the two solstices are two days of equal daytime hours and nighttime hours called the
EQUINOXES – the vernal equinox occurs around March 21 (the first day of spring) and the
autumnal equinox occurs around September 21 (the first day of autumn).
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
The Moon also rotates on its axis. As it rotates, the Moon also revolves around the Earth. The
Moon completes one rotation on its axis in about the same time it takes to complete one
revolution around Earth. The result is that the same side of the Moon faces Earth at all times.
Together, the Earth and Moon pair revolve around the Sun.

ECLIPSES are a darkening of a celestial object due to the position of another celestial object.

SOLAR ECLIPSE occurs because the Sun has a diameter 400 times greater than the Moon. It
is also 400 times farther from Earth than the Moon is. As a result, the Moon and the Sun appear
approximately the same size in the sky. When the Moon is aligned between Earth and the Sun, it
blocks the Sun from being observed from Earth. A solar eclipse is only possible during a new
moon phase and is quite rare.

LUNAR ECLIPSE is when Earth is positioned between the Sun and the Moon, casting a shadow
on the Moon. A lunar eclipse can appear orange or red.
The Force of Gravity

There is a force of attraction between all objects in the Universe with mass known as
GRAVITATIONAL FORCE, or gravity. The greater the mass of an object, the stronger its
gravitational force. The gravitational force of our massive Sun is strong enough to keep Earth in
orbit. Without the Sun’s gravity, Earth would move away from the Sun into space.
Tides – The Pull of the Moon

TIDES are the alternate rising and falling of the surface of large bodies of water, caused by the
interaction between Earth, the Moon, and the Sun.

The Moon’s gravitational force pulls Earth and its oceans toward it. This causes a bulge of water
to form on the side of Earth facing the Moon. As Earth is pulled toward the Moon, a bulge of
water also forms on the opposite side of Earth, where the Moon’s gravitational force is weakest.
This results in two high tides and two low tides on Earth each day. The time between low and
high tides is approximately six hours.
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Patterns in the Night Sky
CONSTELLATIONS

A CONSTELLATION are groupings of stars, as observed from Earth.

Many cultures have noticed that some stars in the night sky appear to form patterns. They began
naming these star patterns after their heroes, mythical monsters, and animals (such as Leo the
lion).
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For example, Ursa Major is the constellation where the Big Dipper can be found.

A star map is a map of the night sky that shows the relative positions of the stars in a particular
part of the sky.
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A CELESTIAL SPHERE is the imaginary sphere that rotates around Earth, onto which all
celestial objects are projected. The celestial sphere extends around Earth, however an observer
can only see half of the sphere (since you cannot see behind you).

CELESTIAL NAVIGATION is the use of positions of stars to determine location and direction
when travelling. The North and South Celestial Poles are the imaginary points where Earth’s axis
of rotation extends out onto the celestial sphere above the North and South Poles on Earth. For
observers in the northern hemisphere, the North Celestial Pole points very close to Polaris (the
North Star).
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A planet can be distinguished from a star in the night sky by observing its motion over weeks or
months.
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In addition, our view of the night sky changes with each passing season due to Earth’s revolution
around the Sun. For example, if we observe the northern hemisphere’s night sky in the winter, we
see stars that are opposite the Sun.
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Stars that are close to Polaris are visible in all seasons. These stars never appear to set, due to
their proximity to one of the celestial poles. For example, Polaris is near the North Celestial Pole
– and in North America, Polaris is visible for the entire night on every night of the year. The
same is true for constellations near the celestial poles. In the northern hemisphere, these
include: Cassiopeia, Cepheus, Draco, Ursa Minor, and Ursa Major.
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RETROGRADE MOTION

RETROGRADE MOTION is the apparent motion of an object in the sky, usually a planet, from
east to west, rather than in its normal motion from west to east.

This apparent motion occurs because Earth travels around the Sun faster than the outer planets.
For example, as Earth passes Mars in its orbit, Mars appears to at first, stop and then travel
backward in the sky. Earth continues past Mars,a nd forward motion appears again in the sky.

We observe retrograde motion of only those planets that are farther from the Sun than Earth.

Each of the outer planets retrogrades for different lengths of time. Jupiter spends 4 of every
13 months in retrograde motion; whereas Neptune retrogrades for 5 months of every year.
NAVIGATING THE NIGHT SKY

In geography, latitude and longitude are used to pinpoint a place or object on Earth.

In astronomy, celestial coordinates are called azimuth and altitude – and are used to describe the
position of a celestial object in the sky relative to an observer on the ground for a particular time
and place.

AZIMUTH is the distance measured from north along the horizon to a point directly below the
celestial object. North has an azimuth of 0°, east has an azimuth of 90°, south has an azimuth of
180°, and west has an azimuth of 270°.
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
ALTITUDE is the angular height a celestial object appears to be above the horizon; it is
measured vertically from the horizon.
SATELLITES

Earth has one natural satellite orbitting it – the Moon.

Earth also has thousands of other satellites circling it at different altitudes and orbits, but
these are made by humans.

Artificial satellites help forecast weather, monitor agriculture, aid in telecommunication or
navigation, assist military activities, and explore the Universe.

Human-occupied spacecrafts, such as the Space Shuttle, and space facilities, such as the
International Space Station, also function has artificial satellites.

In 1957, the first artificial satellite (Sputnik 1) was sent into space by the Soviet Union. Five
years later, Canada’s satellite (Alouette 1) was launched.

Artificial satellites don’t plunge back to Earth because the force of Earth’s gravity continuously
pulls the satellite toward it, yet the forward motion of the satellite and the curvature of Earth
prevent the satellite from getting any closer to the surface.
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
Think of a cannonball – it would fly through the air until it curved toward the ground, pulled down
by gravity. If the cannonball was fired with more velocity, it would travel farther before coming
to rest on the ground. It would still curve toward Earth, but Earth also starts to curve beneath
it because Earth is shaped like a sphere. If we could fire a cannonball with enough velocity, it
would fall toward Earth but never actually hit it because its flight would extend around the curve
of Earth.
TYPES OF ORBITS

Artificial satellites orbit outside Earth’s atmosphere at altitudes of 200 km to more than 35 000
km.

The higher the satellite is, the longer the orbital period – the time it takes to circle Earth.

At an altitude of about 350 km, the International Space Station (ISS) takes 90 minutes to orbit
Earth, whereas Canada’s MOST space telescope satellite completes one orbit 820 km above
Earth’s surface in 101 minutes.

Low Earth Orbit Satellites – Most human occupied spacecrafts and those conducting Earth
observations are set at low-altitude orbits. These low Earth orbit satellites revolve around our
planet at altitudes of up to 2000 km. These satellites have many uses, including military and
Earth observation. Canada’s RADARSAT satellites are in polar orbits to keep an eye on a variety
of natural and human-made events. They chart icebergs in Canada’s far Arctic oceans, monitor
shifting patterns in agriculture in Africa, and play an important role in natural disaster response
in Asia.

Medium Earth Orbit Satellites – travel at altitudes up to 35 000 km. Two dozen of these
satellites are part of the GLOBAL POSITIONING SYSTEM (GPS). GPS satellites travel in
medium Earth orbits at about 11 000 km and aid in navigation by transmitting signals down to GPS
receivers on the ground – providing them with precise geographical coordinates of their location.

One type of satellite orbits Earth at a distance of 35 790 km, which is about a tenth of the way
to the moon. This altitude is significant because it produces an orbital period equal to the period
of the rotation of Earth. When a satellite is orbiting at this height directly above the equator, it
is said to be in GEOSTATIONARY ORBIT. Satellites in geostationary orbit appear motionless
in the sky, which makes them useful for communications and other commercial industries because
they can be linked to antennas on Earth. Weather satellites, for example, track weather in this
manner. Communication industries use geostationary satellites for satellite broadcast television
and radio.
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