|Earth and the night sky 1. Earth's Position in the Milky Way
|How does the Earths position in the Universe, its orbit and rotation determine our view of the night sky;
To appreciate the earth's orbit, the Milky Way can be viewed from an angle perpendicular to the plane of the ecliptic, so the plane of the Milky Way is tilted
away the from the viewer. The view point is shown above, the result below.
|The drawing above illustrates the location of Earth through the seasons. In the summer, the Earth is closest to the Milky Way center, in the winter furthest way.
At midday, midsummer and midwinter, an Earth observer looking at the Sun is aligned with the Sun - Milky Way center axis.
The sky view at midnight , looking directly up, will be used a convenient reference point. At midsummer, midnight looking up, the Earth observer is pointed
towards the Milky Way center. At midwinter, midnight, looking up, the earth observer is pointed away from the Milky Way center.
|The Earth rotates about an axis tilted at 23 degrees from the ecliptic, away from the center of the Milky Way. This can be best seen from a viewpoint looking
along the eclipti, as shown above, with Earth at midwinter and midsummer locations.The Sun - Earth axis is aligned with the Sun - Milky Way center axis.
The Pole Star (Polaris) is a star in the Milky Way above the Earth that happens to be located directly in line with the Earths axis of rotation.
An observer is shown at a latitude of 30 degrees North, roughly Texas. At midsummer, midnight, the observer looks up above the Milky Way plane, so the
center of the Milky Way appears low in the sky, the opposite of the midsummer sun. At midwinter, the edge of the Milky Way appears high in the night sky.
A northern hemisphere viewing location, means that the center, and brightest part of the Milky Way tends to be low in the sky due to the tilt of the Earths
rotation away from the center of the Milky Way. The best view of the center Milky Way will be obtained from anywhere on a latitude 23 degrees South, on
midsummers day at midnight.
Michael P.C. Watts Copy write 2015
|The Milky Way is the largest structure in our night sky. It is spiral collection of 200-400 billion (2-4x10^11) stars, and probably at least as many planets. The spiral
forms a thin disk with an diameter to thickness ratio of 100:1. Stars are an average 0.1 light years apart.
The sun rotates around the Milky Way every 240 M years, so is stationary for human life on Earth.
The Milky Way is our home Galaxy that we view "edge on", as we are located about 2/3 of the way out from the center of the Milky Way. The planets orbit the
sun in a plane (plane of the ecliptic) that is at an angle of 63 degrees to the plane of the Milky Way. The Earth orbits 10^8 km from the sun and the sun is 10^17
km from the center of the Galaxy. To visualize the relationship, the Earth orbit is magnified by 10^9 times in the drawing below.