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Unit 1 - Stars
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In class notes from the first unit of grade 12 earth and space science, stars.
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Unit 1 - Earth & Space Science Notes, The Night Sky
Introduction
● For thousands of years human believed that the Earth was the centre of the Universe, and that
all celestial objects (stars, planets, Moon, and the Sun) revolved around the Earth.
● Ptolemy, a Greek astronomer, proposed a geocentric model of the Universe in the 2nd century
A.D.
● His model postulated that:
○ The stars were located on a large outer sphere and rotated rapidly;
○ The planets were each on smaller inner circular orbits;
○ The Sun revolved around the Earth on the 4th orbital circle from Earth
● It wasn’t until 1543 when Nicolaus Copernicus, a Polish astronomer, published his works in a
book titled “On the Revolutions of Celestial Spheres” that the geocentric model was disproved.
Copernicus’s heliocentric model placed the Sun at the centre, and Earth as one of the planets
that revolve around it. Copernicus made a number of key contributions to astronomy.
The Night Sky
● Have you ever looked up into the night sky and been fascinated by what is out there? Early
astronomers believed that the stars were fixed on a sphere surrounding the Earth. This would
be like having the stars attached to a giant ball around the Earth. Although we now know that
this is not true, it is easier to use this representation to locate stars in the sky.
● The Celestial Sphere is a dome that surrounds the Earth. It is divided in much the same way
as we divide Earth with the North and South celestial poles as an extension of the Earth’s axis.
The celestial equator is a projection of Earth’s equator onto the sphere.
● As the Earth rotates inside the celestial sphere, the stars appear to rise in the east and then
set in the west.
● Meridian
○ The line between exactly north and south, dividing Earth in half
● Zenith
○ The point at which is directly overlooking the middle of the meridian
● Ecliptic
○ The imaginary line that the Sun traces as the Earth revolves around the Sun
● Moon and planets are always seen on/near the elliptic line
● Ecliptic appears more overhead during the winter months
● Celestial pole would be at zenith if you were at the North Pole on Earth
● Celestial Pole
○ Point in the sky around which the stars appear to rotate
● In the Northern Hemisphere, the north celestial pole is near Polaris (The North Star)
● Stars appear as if they are on a “sphere” surrounding the Earth, despite existing at vast
distances from us.
● Right ascension are the lines going away from the celestial pole (similar to longitude)
● Declination are the concentric circles measuring degrees from celestial pole (similar to latitude)
Locating Stars
● Astronomers use a coordinate system similar to the latitude-longitude system used on Earth to
locate stars in the sky.
● There are two different coordinate systems:
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