Introduction to the Night Sky

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Introduction to the Night Sky

• This presentation will review some of the
  important objects that can be viewed in the night
  sky without the aid of a telescope.
• It will review the motion of stars as viewed from
  Earth, both on a daily basis and on an annual
  basis.
• It will also review the concept of a celestial
  sphere and the coordinate system associated with
  it.

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Basic Direction Coordinates

• Probably the simplest way to describe the
  position of an object in the sky is to describe
  its DIRECTION and ELEVATION.
• DIRECTION is usually referenced to magnetic
  North.
• ELEVATION is an angle, from 00 to 900, where 00
  is horizontal and 900 is straight up.
• Example Point to NNE, elevation 600.

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Celestial Coordinates

• To understand the concept of Celestial
  Coordinates, it is helpful to first understand
  the concept of a celestial sphere.
• In our normal experience, the Earth is a fixed,
  non-moving position from which we observe the
  sky.
• Since ancient times, it was noted that the stars
  seemed to rotate each day around a fixed point in
  the sky.

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Celestial Coordinates

• The daily motion of the stars around a fixed
  point is called diurnal or daily motion. It
  can be seen clearly in the picture on the next
  slide.
• The point that stars rotate around also has a
  star located at it. Do you know what it is
  called?

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The Celestial Sphere concept

• To the ancients, it was totally logical to assume
  that the stars were fixed to a distant, invisible
  sphere that rotated around the Earth
  approximately once per day.
• Every star occupied a spot on that sphere, which
  could be described using a coordinate system
  exactly like latitude and longitude.

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The Celestial Sphere
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The Celestial Sphere
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Defining Celestial Position

• In the celestial coordinate system the North and
  South Celestial Poles are determined by
  projecting the rotation axis of the Earth to
  intersect the celestial sphere, which in turn
  defines a Celestial Equator. The celestial
  equivalent of latitude is called declination and
  is measured in degrees North (positive numbers)
  or South (negative numbers) of the Celestial
  Equator. The celestial equivalent of longitude is
  called right ascension.

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Defining Celestial Position

• Right ascension can be measured in degrees, but
  for historical reasons it is more common to
  measure it in time (hours, minutes, seconds) the
  sky turns 360 degrees in 24 hours and therefore
  it must turn 15 degrees every hour thus, 1 hour
  of right ascension is equivalent to 15 degrees of
  (apparent) sky rotation

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Digression - Groups of Stars

• Historically, constellations were groupings of
  stars that were thought to outline the shape of
  something, usually with mythological
  significance. There are 88 recognized
  constellations, with their names tracing as far
  back as Mesopotamia, 5000 years ago.

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Constellations and Asterisms

• Some of the more familiar "constellations" are
  technically not constellations at all. For
  example, the grouping of stars known as the Big
  Dipper is probably familiar to most, but it is
  not actually a constellation. The Big Dipper is
  part of a larger grouping of stars called the Big
  Bear (Ursa Major) that is a constellation.
• A well-known grouping of stars like the Big
  Dipper that is not officially recognized as a
  constellation is called an asterism.

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The Big Dipper is an Asterism found within the
Constellation Ursa Major.
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Stars Look Different from Planets
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Planets

• The term "planet" originally meant "wanderer" it
  was observed long ago that certain points of
  light wandered (changed their position) with
  respect to the background stars in the sky. In
  ancient times, before the invention of the
  telescope and before one understood the present
  structure of the Solar System, there were thought
  to be 7 wanderers or planets Mercury, Venus,
  Mars, Jupiter, Saturn, the Moon, and the Sun.
• Question How is this list different from a
  modern list of planets?

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• The Earth is missing, because it was not
  understood that the points of light wandering on
  the celestial sphere and the Earth on which we
  stood had anything in common.
• Uranus, Neptune, and Pluto are missing because
  they would only be discovered when the telescope
  made them easily visible.
• Uranus is barely visible to the naked eye it was
  discovered in 1781.
• Neptune and Pluto are too faint to see at all
  without a telescope they were discovered in 1846
  and 1930, respectively.
• The Sun and the Moon were classified as planets
  because they wandered on the celestial sphere,
  just like Mars and Jupiter and the other planets.

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Wanderers

• Planets (and the sun and moon) move with respect
  to the background stars. This motion is not
  easily connected to the daily or annual motion of
  the celestial sphere (except for the sun.)
  However, the ancients did notice that the motion
  of the wanderers was essentially always along a
  certain path, which they named the Ecliptic.

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The Ecliptic

• The apparent path of the Sun and planets on the
  celestial sphere is called the ecliptic. Because
  the rotation axis of the Earth is tilted by 23.5
  degrees with respect to the plane of its orbital
  motion (which is also called the ecliptic), the
  path of the Sun on the celestial sphere is a
  circle tilted by 23.5 degrees with respect to the
  celestial equator.

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Defining Zero right ascension

• The zero point for celestial longitude (that is,
  for right ascension) is the Vernal Equinox, which
  is that intersection of the ecliptic and the
  celestial equator near where the Sun is located
  in the Northern Hemisphere Spring. The other
  intersection of the Celestial Equator and the
  Ecliptic is termed the Autumnal Equinox. When the
  Sun is at one of the equinoxes the lengths of day
  and night are equivalent (equinox derives from a
  root meaning "equal night"). The time of the
  Vernal Equinox is typically about March 21 and of
  the Autumnal Equinox about September 22.

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Celestial Coordinates
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The Constellations of the Zodiac

• The zodiac is an imaginary band 18 degrees wide
  centered on the ecliptic. The constellations that
  fall in the zodiac are called the 12
  constellations of the zodiac. They were at one
  time thought to have great mystical and
  astrological significance.
• The constellations of the zodiac are still of
  importance because the planets, as well as the
  Sun and Moon, are always found within one of the
  zodiac constellations.

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Connecting the sky and the Earth

• As you saw in the previous slide, the important
  positions where the celestial equator and the
  ecliptic intersect are connected to important
  moments on Earth during the year. By noting the
  position of the sun, moon, and stars using stone
  circle (or similar) calendars, people made a
  connection between positions of objects in the
  sky and seasonal events on Earth.

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Annual Motion

• Each day, the stars return to nearly the same
  place they were the previous night. But aver
  time, the positions drift until a star that was
  once high in the sky at a certain hour sets on
  the Western horizon and eventually becomes
  invisible during the night sky. The star
  reappears on the Eastern horizon later in they
  year (at the same hour) and eventually returns to
  its original spot in the sky.

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Annual Motion

• Because this process takes about one year, it is
  called annual motion. We now understand that
  the change is caused by Earths orbit around the
  sun, but to the ancients, it was the SUN moving
  independently from the rest of the Celestial
  Sphere as the Earth remained in a fixed position.
• Lets set up a classroom demonstration.