DSST Astronomy

# The earths axis is tilted from the vertical by 235 as

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The Earth's axis is tilted from the vertical by 23.5° as it makes its orbit around the sun. The result of this tilt (or obliquity of the ecliptic) is that the Northern Hemisphere is inclined toward the sun in June and away from it in December, thus varying temperatures in the seasons: An Equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator. Vernal equinox =spring; is defined by the intersection of the ecliptic and the celestial equator. Solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky reaches its northernmost or southernmost extremes. The summer solstice is the longest day of the year, and the first day of summer. The winter solstice is the shortest day of the year, and the first day of winter. The changes to spring and autumn are called equinoxes . Question: After the Summer Solstice is over the sun will appear to be Lower in the sky each day. 7

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ASTRONOMY DSST 05Mar2012 The horizontal coordinate system uses measurements relative to the observer's horizon, therefore two people standing in two different parts of the world will have different measurements for the same star. The system consists of two measurements--the azimuth and the altitude . The azimuth runs from 0° to 360° and tells the observer what direction to look in, while the altitude tells the observer how high to look. There were @ 2 questions in regards to the above diagram. Ie…where would you be standing if you observed a star at a certain angle. (Good luck with that) Parallax - This is the apparent shift of an object against the background that is caused when the observer changes positions. When standing in one place it looks one way; if the observer moves, the background changes and the object looks different. The below graphic shows a simple demonstration of how the same star would look different when the Earth is in two different parts of its orbit around the sun. 8
ASTRONOMY DSST 05Mar2012 Galaxies: Clusters - The cluster of galaxies to which our galaxy belongs is called the Local group . Spiral - Milky Way (our solar system) A spiral consists of a nucleus, a disk, a corona, and spiral arms. Elliptical - (Dwarf Galaxy) Elliptical galaxies are spherical or ellipsoidal systems which consist entirely of stars with no trace of spiral arms. They have a much greater range in size, mass, and Luminosity than do the spirals. No disks visible which means slower rotation. Irregular - (3% of galaxies) They show no trace of circular or rotational symmetry. Their appearance is irregular and chaotic. Radial velocity of a galaxy is its velocity in the line of sight, or in other words its speed straight towards you or away from you. Evidence from the radial velocities of the galaxies acknowledges that the universe is expanding. The first radial velocities of galaxies were measured by V. M. Slipher at the Lowell Observatory.

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