Download Basic Astronomy Note - Mr. Dewey – Grade 7/8

Basic Astronomy Note
When you look up at the night sky, all of the stars you see are suns - many of which are much larger than
our sun – but so incredibly far away they appear to us as tiny points of light. Stars are bodies of gas
molecules (hydrogen atoms compressing and fusing into helium). This is nuclear fusion and the heat and
light that results is sunshine. Stars can have different colours (blue, red, yellow), which depends on
temperature. Our sun is a yellow star.
Constellations (e.g., The Big Dipper, Orion) are ancient means of making sense of the night sky. The stars
associated with each constellation are not connected with each other. Some stars are closer, some
further away, but from our point of view they make a familiar pattern.
Some stars are grouped together and these groupings are called clusters. A prominent example of this in
our winter night sky is the Pleiades. To the unaided eye it looks like a ‘fuzzy patch’ of light. Through a
telescope approximately 7 separate stars are visible close to each other.
Latitude determines which stars are visible. Our viewpoint of space is different from the viewpoint of
space at latitudes in the southern hemisphere. For example, at our northerly latitude we can’t see the
stars of The Southern Cross constellation which are visible in Australia.
The nighttime view of the night sky changes throughout the year as well as Earth moves around the sun.
We see different constellations in the summer and winter. However each February 25th at 9:00pm for
example we see the same night sky view because we are in the same place in space in our movement
around the sun.
The planets move around the sun like cars on a racetrack in different lanes, sometimes next to each
other and sometimes passing each other. So we see planets at different times in the night sky from year
to year. Planet comes from the Greek word for “wanderer.” Planets appear as stars in the night sky, but
are really reflecting sunlight and not making light as do stars.
The closest stars to our sun are Alpha and Proxima Centauri, visible only in the southern hemisphere.
They are approximately 4 light years away. At the speed of light it would take 4 years to get there. One
light year is 9.46 trillion kilometers. With current technology, travelling at approximately 50,000km/hr, it
would take about 80,000 years to get there (our closest neighbouring solar system).
Our Solar system
Mercury, Venus, Earth Mars:
These are the rocky planets with solid surfaces. Mars, for example, is further from the sun and so it’s
colder and the sun would appear smaller in the sky.
Jupiter, Saturn, Uranus, Neptune:
These are gas giants with no solid surface although there may be rocky cores. If the Earth were a golf
ball, Jupiter would be a basketball. The Great Red Spot on Jupiter is a hurricane that has been raging for
hundreds of years. Three Earth’s could fit inside the storm.
Motions
The Earth rotates on its axes once per day. The sun, moon, stars and planets all appear to rise in the east
and set in the west. That apparent motion is really the result of the earth’s rotation. We’re moving,
turning into the sunshine and out of the sunshine for example.
The North Star, however, is stationary. If you were at the North Pole, the North Star would be directly
overhead. So, as the earth rotates it’s stationary. The North Star can be thought of as the projection of
the North Pole onto the night sky. As the earth rotates, northerly stars therefore appear to rotate
around the North Star.
The sun, moon and planets are all found along roughly the same line the sky from east to west called the
ecliptic.
The earth orbits the sun once per year. The Earth orbits the sun while maintaining a constant tilt of 23.5
degrees. The tilt results in our seasons. When the northern hemisphere is tilted towards the sun, the
sun’s rays are very direct and so this is our summer. At the same time, the southern hemisphere is tilted
away, and so it’s winter in the countries of that hemisphere. In winter, the sun’s rays aren’t very direct
and we also spend less time in the sunshine as the earth turns as you can see from the diagram below.
Seasons Diagram
As the earth rotates in the summer months of the northern hemisphere, places in the far north never
turn out of the sunshine. The sun appears to circle the horizon. It’s daylight 24 hours a day. Likewise, at
the same time of year (June), locations deep in the southern hemisphere remain in the dark 24 hours a
day, never turning into the sunshine as the Earth rotates.
Days are longer during the summer months (our time in the sunshine is longer during each rotation of
the Earth), and days are shorter during the winter months (our time in the sunshine is shorter during
each rotation of the Earth).
On the first day of summer (summer solstice), the sun at noon is directly overhead at locations along the
Tropic of Cancer (the line on the globe 23.5 degrees north of the equator). This is the furthest north that
the sun’s direct rays reach in our movement around the sun. For reference, the North Pole is considered
90 degrees (North Latitude) and the equator is 0 degrees. Likewise, our winter solstice is when the sun is
most direct (overhead at noon) at locations along the Tropic of Capricorn (the line on the globe 23.5
degrees south of the equator) in the southern hemisphere. On the days after our summer solstice the
sun’s most direct rays fall a little bit further south of the Tropic of Cancer. And so people often say on
the first day of summer, “The days are now getting shorter.” In the days after our winter solstice we can
also say, “The days are now getting longer.”
Phases of the Moon
The moon orbits the earth once per month. It’s 385,000 km away. Like the Earth, half the moon is lit by
the sun. As the moon travels around the Earth, our view of that lit half changes from week to week.
These are the phases of the moon. Each phase, if you break it into four, is a week apart.
Another thing to keep in mind when viewing the moon is that it rises and sets (due to Earth’s rotation)
just like the sun. From night to night however, the moon will rise later and later. If high in the sky at
certain time, it will be further to the east the following night at the same time. This is because the
moon’s movement around the Earth is west to east from our point of view looking up.
Also, here in the northern hemisphere we see the First Quarter moon as the right side of the moon lit
up. If you were in Australia on the same day at nightime, you would see the moon as the left side lit up.
Imagine putting a pie plate, half painted white, on the ceiling. From one side of the room you see the
painted side as the right side, and on the other side of the room you see the painted side as the left side.
It’s just a different perspective on the same object. In the southern hemisphere, you would see the
moon (and sun) in the sky looking north, and here we see both in the sky looking south.
Eclipses
In a solar eclipse, the moon blocks sunlight in a very small section on the daytime side of the Earth.
Because the earth is turning, this shadow moves across continents affecting only those specific local
places that will fall under the shadow spot.
In a lunar eclipse, the moon passes through the shadow that the Earth casts into space. When this
happens, the visible moon is temporarily eclipsed by darkness for a few minutes and then gradually
reappears. Most of the time the moon passes above or below the Earth’s shadow giving us a full moon
every month without an eclipse.
Nebula: a nebula is a vast area of gas molecules suspended in space which glows when heated by
nearby stars. Nebula are ‘nurseries’ for the formation of new stars. Gas molecules compress, fusion
starts, and new stars are born.
Galaxies
Our sun is one star of billions that make up our galaxy (Milky Way). It is 100,000 light years in diameter,
which means it would take 100,000 years to travel from edge to edge at the incredible speed of light
(roughly 300,000 kilometers per second). The Milky Way, that bright band of light in the night sky, is a
cross sectional view of the galactic disk. There are billions of other galaxies with billions of stars (suns) in
each one. The Andromeda Galaxy is our closest neighbor. In clear, dark sky conditions Andromeda can
be seen to the north as a faint ‘fuzzy patch’ of light. It’s 2.5 million light years away, and that ‘fuzzy
patch’ of light is the combined light of billions of suns reaching our eyes from that incredible distance.
Andromeda is the furthest object visible with the unaided eye.