Charting the Heavens

Standards Identify & apply measurement techniques Understand the scale & contents of the universe Predict changes in the positions and appearances of objects in the sky (e.g., moon, sun) based on knowledge of current positions and patterns of movement (e.g., lunar cycles, seasons)

Our Place in Space We live on an ordinary rocky planet that orbits an average star, near the edge of a huge collection of stars, the Milky Way Galaxy, which is one galaxy among countless billions of others spread through the universe.

Our Place in Space The universe is the totality of all space, time, matter and energy Astronomy is the field of science that is concerned with the study of the universe

Our Place in Space In order to study the universe, we have to consider things on scales that are unfamiliar from our everyday experience For example: a common measure of distance is the light year – the distance traveled by light in one year The speed of light (denoted by the letter c) is approximately 300,000 km/s or 186,000 mi/s Therefore, the distance of a light year is 10 trillion km (6 trillion mi)

The Obvious View: Constellations in the Sky We can see ~3000 stars from each hemisphere (6000 total) with the unaided eye.

The Obvious View: Constellations in the Sky It is a natural human tendency to see patterns & relationships between objects even when no true connection exists

The Obvious View: Constellations in the Sky Long ago people connected the brightest stars into configurations called constellations, which were named after mythological beings, heroes and animals The stars that make up a constellation are NOT actually close to one another, and if viewed from a different location in space, they would not form the same pattern

Orion Distances between the stars of Orion

The Obvious View: Constellations in the Sky Early astronomers studied the constellations for navigation, and as calendars to predict planting and harvesting seasons In all, there are 88 constellations

The Obvious View: The Celestial Sphere If you were to watch the sky over the course of a night, the constellations would seem to move from east to west.

The Obvious View: The Celestial Sphere Ancient astronomers were aware that, though the stars moved across the sky, their relative positions to each other remained unchanged They concluded that the stars must be firmly attached to a shell, or celestial sphere, surrounding Earth

The Obvious View: The Celestial Sphere Today, we know the motion of the stars is due to the spin, or rotation, of the Earth on its axis Even though we know the idea of a celestial sphere is incorrect, we use the idea as a convenient model to help us visualize the position of stars in the sky

The Obvious View: The Celestial Sphere Celestial poles – the points where Earth’s axis intersects the celestial sphere Celestial equator – midway between the north and south celestial poles, represents the intersection of Earth’s equatorial plane with the celestial sphere

Image: http://www. astro. columbia sphere.jpg

The Obvious View: Celestial Coordinates There are two methods of locating stars in the sky The simplest method is to specify their constellation and then rank the stars in it in order of brightness The brightest star is denoted by the Greek letter alpha (α), the second brightest by beta (β), etc.

The Obvious View: Celestial Coordinates For more precise measurements, astronomers use a celestial coordinate system

The Obvious View: Celestial Coordinates The latitude and longitude coordinate system that’s used on Earth is extended out to the celestial sphere Declination (Dec) – the celestial equivalent of latitude. It is measured in degrees north or south of the celestial equator. The celestial equator has a declination of 0, the north celestial pole is +90, and the celestial south pole is –90

The Obvious View: Celestial Coordinates Right ascension (RA) (the celestial equivalent of longitude) is measured in units called hours, minutes and seconds, and it increases in the eastward direction Units of RA are constructed parallel to units of time

Image: http://www.onr. navy.mil/focus/space sciences/images/ observingsky/celestial sphere.jpg

Earth’s Orbital Motion: Day-to-Day Changes We measure time by the sun Solar day – the 24 hour day that is our basic unit of time. It is measured from one noon to the next Diurnal motion – the daily progress of the sun & other stars across the sky

Earth’s Orbital Motion: Day-to-Day Changes The stars’ positions in the sky do not exactly repeat from one night to the next Each night, the whole celestial sphere appears to be shifted a little relative to the horizon

Earth’s Orbital Motion: Day-to-Day Changes A day measured by the stars is called a sidereal day It differs in length from a solar day The difference between a solar day and a sidereal day is a result of Earth’s rotation on its axis along with its revolution about the sun The solar day is 3.9 minutes longer than the sidereal day (which is 23h56m long)

Earth’s Orbital Motion: Seasonal Changes The stars/constellations that we see in the sky are different in summer than in winter This is because of Earth’s revolution around the sun Earth’s dark side faces a slightly different direction in space each night

Earth’s Orbital Motion: Seasonal Changes Because of Earth’s motion, the sun appears (to an observer on Earth) to move relative to the background stars over the course of a year This path is called the ecliptic, and it is the plane of Earth’s orbit around the sun Zodiac – the 12 constellations through which the sun passes as it moves along the ecliptic

Graphic: http://www.astrologyclub.org/articles/ecliptic/ecliptic2.gif

Earth’s Orbital Motion: Seasonal Changes Why do we have seasons? Student answers We have seasons because Earth is tilted on its axis. The northern hemisphere faces towards the sun in summer, so the suns rays are more direct, and heat the Earth more. The northern hemisphere faces away from the sun in winter, making the suns rays more glancing and less efficient at warming the Earth

Image: http://www.ifa.hawaii.edu/~barnes/ast110_06/ea/0427a.jpg

Earth’s Orbital Motion: Seasonal Changes Summer solstice – point on the ecliptic where the sun is at its northernmost point above the celestial equator This is the point in Earth’s orbit where the north pole points closest to the sun This occurs on or near June 21, and corresponds to the longest day of the year in the northern hemisphere (shortest day in the southern hemisphere)

Earth’s Orbital Motion: Seasonal Changes Winter solstice – point on the ecliptic where the sun is at its southernmost point Occurs on or near December 21, and corresponds to the shortest day of the year (in the northern hemisphere)

Earth’s Orbital Motion: Seasonal Changes Equinoxes – the two points where the ecliptic intersects the celestial equator On these dates, day and night are of equal length

Earth’s Orbital Motion: Seasonal Changes Autumnal equinox – fall: on or near September 21 The sun crosses from the northern to the southern hemisphere Vernal equinox – spring: on or near March 21 Sun crosses celestial equator moving north

Image: http://www. ecology

Earth’s Orbital Motion: Seasonal Changes Tropical year – interval of time from one vernal equinox to the next = 365.2422 mean solar days This is the year that our calendar measures

Earth’s Orbital Motion: Long Term Changes Earth spins on its axis, travels around the sun and moves with the sun through our galaxy

Earth’s Orbital Motion: Long Term Changes Precession – Earth’s axis changes direction over the course of time Caused by torques (twisting forces) on Earth due to the gravitational pulls of the moon and sun These forces affect our planet in a way that is similar to the way the torque due to Earth’s gravity affects a top A complete cycle of precession takes about 26,000 years and traces out a cone

Image: http://www.lcsd.gov.hk/ CE/Museum/Space/Education Resource/Universe/framed_e/ lecture/ch03/imgs/precession.jpg

Earth’s Orbital Motion: Long Term Changes Sidereal year – time required for Earth to complete exactly one orbit around the sun relative to the stars = 365.256 mean solar days The sidereal year is ~20 minutes longer than the tropical year The reason for this difference is precession

The Measurement of Distance Triangulation – distance-measurement method based on principles of Euclidian geometry Forms foundation of family of distance-measurement techniques that make up the cosmic distance scale

The Measurement of Distance If we know the value of one side of a triangle (its baseline) and two of its angles, we can calculate the lengths of the remaining sides of the triangle Close objects are measured this way using parallax – an object’s apparent shift relative to some more distant background as observer’s point of view changes Earth’s orbit is used as the baseline to measure the angle through which line of sight to an object shifts

Image: http://www.astro.gla.ac.uk/users/kenton/C185/parallax.gif