|Earth and the night sky 2. The hidden Milky Way
|How does the Earths position in the Universe, its orbit, and rotation determine our view of the night sky?
|The "Universe" viewed from the Sun is a 360 sphere. As with maps of the Earth, the challenge is to represent the sphere on a flat surface. A
popular astronomical projection is the homolographic projection, invented by Mollweide in 1805.The projection is centered on the Milky Way
with equal areas on the sphere are given equal areas on the projection.
The center of the projection is the center of the Milky Way, and the central axis is the edge view of the Milky Way from the Sun. The greatest
distortion occurs for structures away from the center on Earth - Sun axis.
|An observer in the northern latitudes has parts of the center of Milky Way hidden, as shown below.
|The latitude of the observer on the earth controls the visibility of the Milky Way. The effect of latitude is best visualized through the
plane of the ecliptic, perpendicular to the tilt axis. The north-south tangent defines the horizon for an observer at roughly 30
degrees north, and is shown above, for Earth at midsummer and midwinter. The northern horizons overlap consistent with North
Star being always visible. The southern horizons diverge, leaving a section of the Milky Way obscured, marked with a purple
triangle. The patch on the projection above shows the "invisible" section of the night sky for a 30 degree north observer. Only an
observer at the equator gets an complete view of the night sky.
The drawing above illustrates the position of the Earth at midnight on midsummer and midwinter. The drawing below shows how
the view for an Earth observer looking up at midnight, changes through the year.
|For a Earth observer, the night sky rotates around the Pole Star, Polaris, situated in Ursa Minor in the left hand side of the projection.
Starting at midwinter, in Dec and Jan, the center of view at midnight for the Earth Observer is pointed at the edge of the Milky Way, away
from the center, the downward tilt of the Earth rotation offsetting the upward tilt of a location in the Northern hemisphere. Through the
spring, their view moves through the sky high above the Milky Way plane out to the right sky. At midsummer, the view is centered on
Serpens Cauda, above the center of the Milky Way, as the tilt from rotation of the Earth and upper hemisphere location combine. In the fall,
the view, is centered just below the Milky Way plane, looking out to the left of the center.
The latitude of an Earth observer limits the view of the opposite hemisphere of the sky. The only view of the entire Milky Way can be seen
from the equator. The tilt of the Earth's rotation away from the center of the Milky Way limits the view of the sky below the Milky Way, and
provides a much better view of the left hand sky from the northern hemisphere.