|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;
1 Earth orbital motion in the Milky Way
2) The hidden Milky Way
3) The changing view of the Milky Way overnight
4) Position of the Milky Way in the universe
5) Best viewing of astronomical highlights
6) Scale of the universe
To understand the view from the earth, 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, midwinter, 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 ecliptic, perpendicular to the tilt, 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 to the 11th power) 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 to the
8th power km from the sun and the sun is 10 to the 17th power km from the center of the Galaxy. To visualize the relationship, the Earth orbit is
magnified by 10 to the 9th power times in the drawing below.