Celestial Coordinates and Earth's Motion

• Astronomy uses two main systems to locate objects in the sky

Horizon Based: Altitude/Azimuth

• Azimuth measures around the local horizon, starting at North and going East, South, and West
• North = 0° or 360°
• East = 90°
• South = 180°
• West = 270°
• Altitude measures from the horizon straight up to the object
• 0° is on the horizon
• 90° is straight overhead

Equator Based: Right Ascension/Declination

• Right ascension measures around the equator, starting at the point where the equator crosses the ecliptic
• Declination measures from the equator up (or down) to the object
• 0° is on the equator
• +90° is at the north pole
• -90° is at the south pole

Daily Motion of Earth

• Earth rotates on its axis counter-clockwise when viewed from above the north pole
• The spin makes everything in the sky appear to rotate around the earth
• Most objects rise and set
• Locations in the northern hemisphere have objects that remain above the horizon full time, known as north circumpolar objects
• Solar noon occurs when the sun crosses the meridian, which is the middle of the sky at Azm 180°
• The average time from one solar noon to the next is 24 hours and varies slightly throughout the year
• A star to travel from the meridian around again needs 23h 56m 4.01s, this is called the Sidereal Day
• The difference between mean solar day and sidereal day is because Earth orbits around the sun
• Each day, Earth moves about 1° around its orbit
• After rotating 360°, Earth is pointed 1 degree away from the sun, requiring one more degree of rotation to reach solar noon
• Observing the same star at the same clock time each day results in it appearing about 1 degree further west
• Over a full year, a star will appear to move 360 degrees around the sky

Annual Motion of Earth

• Earth revolves around the sun (the orbit) once per year
• The axis of Earth's spin and the axis of its orbit are misaligned by 23.5 degrees
• This tilt causes annual swings in the energy received from the sun
• Northern latitudes receive maximum energy on the summer solstice (June 21)
• Northern latitudes receive minimum energy on the winter solstice (December 21)
• Southern latitudes have reverse seasons
• On the Vernal and Autumnal Equinox (March 21 and September 21), day and night are roughly equal
• The sun is directly overhead at the Equator on the equinoxes
• The sun is directly overhead at the Tropic of Cancer on the summer solstice
• The sun is directly overhead at the Tropic of Capricorn on the winter solstice
• The arctic circle is the furthest south location that experiences at least one day without a sunrise and one day without a sunset
• The antarctic circle is the opposite
• The axis of the tilt (and the location in the sky of the equinox) precesses once every 25,700 years
• As a result of the precession of the equinox, the north star will not always be at the pole and will move by 2°-3° in one's lifetime

Motion of The Moon

• The Moon orbits around the Earth
• The amount of the moon illuminated varies each day
• Half of the moon is always illuminated, our view of depends on the angle
• It takes 27.25 days for the moon to orbit 360 degrees around the earth and is known as a sidereal month
• It takes 29.5 days for the moon to complete the cycle of phases and is known as a synodic month
• The difference in sidereal and Synodic month lengths is caused by Earth's orbit around the sun, moving 13° during each lunar cycle
• The moon is tidally locked with Earth, so the same side faces Earth at all times, requiring the moon to spin exactly once per orbit
• The plane of the moon's orbit is not exactly aligned with the plane of Earth's orbit
• It is off by 5.14° and is sometimes above or below the plane of Earth's orbit
• When the moon crosses the plane of Earth's orbit during a full or new moon phase, an eclipse occurs
• A solar eclipse occurs when the moon passes directly between Earth and the sun
• A Lunar eclipse occurs when the moon passes directly behind Earth, into Earth's shadow

Keeping time

• The motion of the Earth, sun, and moon form the basis for our timekeeping system
• The mean solar day is divided into 24 hours
• Each hour is divided into 60 minutes
• Each minute is divided into 60 seconds
• If seconds are divided, they are divided into decimals
• Modern science has defined the second precisely and built up from there
• A mean solar day is 24 hours
• A Sidereal day is 23h56m4.01s
• The tropical year, or the time from one equinox to the same equinox again, is 365.2422 days
• This is our usual year
• The sidereal year (the time for one exact 360° orbit) is 20 minutes longer due to the precession of the equinox, totaling 365.256 days
• A lunar year (12 lunar months) is 354 days
• A lunar month is one lunar cycle
• Calendar months do not correspond exactly to a lunar month because the number of days and months are uneven, with all but February being longer

Planets And Other Celestial Objects

• Our solar system has objects primarily under the sun's gravity, including planets, dwarf planets, moons (satellites), and small solar system objects Visible Planets
• Mercury, Venus, (Earth), Mars, Jupiter, Saturn are the five planets can be seen in the sky without a telescope
• Uranus and Neptune are only visible with a telescope
• There are several dwarf planets, like Ceres and Pluto, that are smaller and haven't cleared their orbit
• Mercury, Venus, Earth, and Mars are mainly made of rocky materials and are considered the inner solar system
• Jupiter, Saturn, Uranus, and Neptune are much bigger and have more gas and ice and are considered the outer solar system
• Small solar system bodies may be rocky or icy, with elliptical or circular orbits
• System bodies may have different designations based on their orbital characteristics
• Small solar system bodies with circular orbits are called asteroids
• An icy small solar system body with a very elliptical orbit is often called a comet
• Small solar system bodies trapped by a planet’s gravity but not orbiting the planet are called Trojans
• Small solar system bodies between Jupiter and Neptune are called Centaurs, and when affected by a planet's gravity, they can be pushed toward the sun to become a comet, move to a new centaur orbit, or be ejected from the solar system

Constellations

• Constellations are groups of stars near each other in a region of the sky
• Ancient astronomers and astrologers used constellations for calendars, navigation, storytelling, and fortune-telling
• Modern astronomers use constellations to divide up the sky by filling in the gaps in the Classical Greek/Roman constellations
• Different cultures have different constellation groupings based on the same or different asterisms
• Pleiades is a common asterism recognized by nearly all cultures with stories that involve seven children (sisters or kids)
• Constellations along the sun's path (the ecliptic) are called the Zodiac, useful for calendar-building and navigation, which include
  • Aries
  • Taurus
  • Gemini
  • Cancer
  • Leo
  • Virgo
  • Libra
  • Scorpio
  • Sagittarius
  • Capricorn
  • Aquarius
  • Pisces
• In northern latitudes, a group of constellations near the celestial north pole will always be above the horizon, circling the celestial north pole. These are north circumpolar constellations and include
  • Ursa Major (Big Dipper)
  • Ursa Minor (Little Dipper)
  • Draco
  • Cassiopeia
  • Cepheus
  • Camelopardalis
• Orion is an easy-to-spot constellation, near Canis Major, Taurus, and Gemini
• Looking in the summer sky for The Summer Triangle which can find Cygnus, Aquila, and Lyra
• Circumpolar constellations can help in telling, directions, latitude, and time
• Latitude can be easily calculated by measuring the altitude of the celestial pole
• "Altitude of polaris" = "latitude on the globe"
• Longitude is harder to tell from the sky without a clock and can be done by comparing rise, set, and transit times from a home base to a current location

Explore celestial coordinate systems, including altitude, azimuth, right ascension, and declination. Understand circumpolar stars and the relationship between sidereal and solar days. Investigate the impact of Earth's axial tilt on seasons.