Discovering the night sky

Patterns of Stars When you stargaze, without too much light pollution, the unaided human eye can only detect about 6000stars over the entire sky. At any one time, you can see roughly 2000 stars (those above the horizon – the boundary between Earth and sky) Smoggy or light polluted areas – only a tenth of that

These patterns of stars, formed by the brightest of the stars, are formally called asterisms. You are more familiar with the common names, such as the Big Dipper or Orion. These are informally called constellations and have names derived from ancient legends.

Cancer A crab that was sent to pinch and distract Hercules while he was fighting the Hydra

Leo Hera ordered creation of lion Hercules strangled the lion after it killed people and cattle

Orion Orion was a great hunter Feel in love with Artemis – moon goddess Killed by Scorpius – and placed in the night sky opposite so he can’t see Scorpius

Scorpius Scorpion sent by Gala to battle Orion – she felt like he was killing too many animals

Ursa Major Arcas – illegitimate son of Zeus. Zeus turned him into a bear after he tried to kill his mother

Ursa Minor Callisto – who was turned into a bear by Hera

Polaris (the North star) Found from the “pointer stars” on the Big Dipper, and on the “handle” of the Little Dipper, it is located right over the North Pole. Polaris is considered a “moderately bright” star, but is not even among the 20 brightest. But face Polaris, and you are facing north.

Brightest Star in the Sky The brightest star in the night sky is Sirius. Locate the belt of Orion, and follow a straight mental line to the left. Hand out table of brightest stars

The Celestial Sphere Astronomers have created a “sky map” called the celestial sphere, and applied a coordinate system to it. This system is analogous to north/south and east/west of latitude and longitude If you know a star’s celestial coordinates, you can locate it quickly.

Constellations The celestial sphere is divided into 88 constellations of different sizes and shapes. The boundaries of constellations are straight lines that meet at right angles. Hand out list of constellations

So while asterisms are called constellations in normal conversations, astronomers use the word constellation to describe an entire area of the sky and all the object in it.

Celestial Sphere, cont… Stars only seem fixed on the sphere because of their remoteness – in reality, they are at widely varying distances from Earth, and they DO move relative to one another But they are all more than 25 trillion miles from us; so although they may change, the changes over the course of a human lifetime are insignificant Therefore, the celestial sphere is a great tool for navigating the heavens

So declination (dec) is equivalent to latitude, and right ascension (ra) is like longitude.

Earth’s rotation rotation – Earth on its axis; creates day and night diurnal motion – daily motion of celestial bodies (can see through time exposure) revolution – motion of any astronomical object around another astronomical object sidereal period – length of time it takes for any cycle of motion (for Earth around the sun it’s 365 ¼ days)

http://www.gemini.edu/gallery/v/Special-Images/Video/GSExtremeStarTrails.jpg.html Circumpolar star trails – long exposure aimed at the South Pole – any stars that pass between the pole and the ground are circumpolar – they never go below the horizon.

Stars at celestial equator http://images.fineartamerica.com/images-medium-large/star-trails-of-the-celestial-equator-luis-argerich.jpg Stars at celestial equator

Seasons Both the tilt of the Earth’s axis and the revolution around the Sun result in the Earth having seasons The Earth’s rotation axis tilt is exactly 23½◦

So what differences do you see at equinoxes and solstices? Sun is lowest in the northern sky in winter solstice – as the Sun moves north, the amount of daylight increases When the sun reaches the summer solstice, it is highest in the northern sky and is above the horizon for the most number of hours

At latitudes above 66½° north or below 66½° south, the Sun does not rise at all during parts of fall or winter – during spring and summer, they have continuous sunlight for weeks or months.

Eclipses and Phases lunar eclipse – when the Moon passes through the Earth's shadow Moon often darkens to deep red in color Only happens when Sun, Earth, and Moon are in a straight line at a full Moon solar eclipse – when the Moon’s shadow moves across the Earth’s surface Broad daylight turns into twilight Moon moves in front of the Sun

Conditions for Eclipses Although it seems like they should occur every new and full moon, they happen much less The moon’s orbit is tilted 5° from the ecliptic, so the full and new moons often occur when the Moon is above or below the Earth’s orbit Moon crosses the ecliptic at lines of nodes Happens so rarely – eclipses are about 2-5 every year, no more than 7 combined handout

Three types of Lunar Eclipses Earth’s shadow has two distinct parts umbra – part of the shadow where all light is blocked by the Earth penumbra – Earth blocks only some of the sunlight

If you were in the umbra looking at Earth, you couldn’t see the Sun. If you were in the penumbra looking at Earth, you would see a crescent Sun. How the Moon travels through Earth’s shadow, results in the type of lunar eclipse formed.

Why red? A small amount of sunlight passing through Earth’s atmosphere is bent into the umbra. The light deflected is primarily in the red and orange end of the spectrum, and so the darkened moon will glow in that color.

Three types of Solar Eclipses The moon’s shadow also has an umbra and a penumbra region. So depending on where the Earth is in that shadow, that is the type of eclipse formed.