The Sky

Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups.

Constellations (2) They were believed to represent great heroes and mythological figures. Their position in the sky seemed to tell stories that were handed down from generation to generation over thousands of years.

Constellations (3) Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

Asterisms An asterism is part of a constellation or part of several constellations. The Big Dipper is part of Ursa Major The summer triangle is part of Lyra, Cygnus, and Aquila The great square is part of Pegasus and Andromeda

Constellations (4) The stars of a constellation only appear to be close to one another Usually, this is only a projection effect. The stars of a constellation may be located at very different distances from us.

Constellations (5) Some examples of easily recognizable constellations and their brightest stars

Constellations (6) Stars are named by a Greek letter (  ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Orion Betelgeuze Rigel Betelgeuse =  Orionis Rigel =  Orionis

Star Names Stars are named by a Greek letter (  ) and Latin Constellation name. The naked eye visible stars have ancient names, usually Arabic. Stars discovered today are given Modern Star Numbers. If you discover a star, you do NOT get to name it.

The Apparent Magnitude Scale First introduced by Hipparchus ( B.C.): Brightest stars: ~1 st magnitude Faintest stars (unaided eye): 6 th magnitude More quantitative: 1 st mag. stars appear 100 times brighter than 6 th mag. stars 1 mag. difference gives a factor of in apparent brightness (larger magnitude => fainter object!)

The Apparent Magnitude Scale Sirius (brightest star in the sky): m v = Full moon: m v = Sun: m v = The magnitude scale system can be extended towards negative numbers (very bright) and numbers greater than 6 (faint objects):

The Apparent Magnitude Scale Apparent magnitude is not the only type. Consider the stars Sirius and the Sun. Sirius puts out much more energy than the Sun. So how is it that the Sun is so much brighter? Of course it is because the Sun is so much closer to us. It is, therefore, APPARENTLY brighter. If the 2 stars were at the same distance from us, Sirius would appear brighter.

The Absolute Magnitude Scale The ABSOLUTE magnitude scale is based on all stars being 10 parsecs away. (Obviously we can’t physically move the stars, but we compare them as if they were 10 parsecs away.) In this case, you can see that Polaris is the brightest of the three stars and the sun the dimmest.

The Absolute Magnitude Scale At 10 pc

Apparent and absolute magnitudes of common stars Object m M (apparent) (absolute) Sun Sirius Vega Betelgeuse Polaris

The Celestial Sphere Not real, but a useful concept. A sphere surrounding Earth with the stars stuck to it. Everything revolving around Earth.

Celestial Sphere Imagine a person on Earth. What can they see? Where you stand gives you different views. You can only see half of the celestial sphere at any one time from any place on Earth. What you see depends on time of day/night, location on Earth, time of year.

The Sun and Its Motions The Sun’s apparent path on the sky is called the Ecliptic. Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere. Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations.

Terms Zenith – The point on the celestial sphere above the observer’s head. Horizon – The great circle between the zenith and nadir. The circle on the celestial sphere cut by a plane tangent to the Earth at your feet. Everything in the sky above the horizon is visible, while everything below it is not. Nadir - The point on the celestial sphere directly underneath the observer’s feet. (not visible!)

The Celestial Sphere Celestial equator = projection of Earth’s equator onto the c. s. North celestial pole = projection of Earth’s north pole onto the c. s. Zenith = Point on the celestial sphere directly overhead Nadir = Point on the c.s. directly underneath (not visible!)

Distances on the Celestial Sphere The distance between two stars on the celestial sphere can only be given as the difference between the directions in which we see the stars. Therefore, distances on the celestial sphere are measured as angles, i.e., in degrees ( o ): Full circle = 360 o arc minutes (‘): 1 o = 60’ arc seconds (“): 1 ’ = 60”

Two systems: –Global Right Ascension (like longitude) Declination (like latitude) –Local Altitude Azimuth Finding things in the sky

Celestial Coordinates Declination: degrees north or south of celestial equator Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox

Altitude and Azimuth Altitude = How high up off the horizon. Measured in degrees, from 0 to 90. Azimuth = Which direction along the horizon. Measured in degrees, from 0 to 360.

Altitude and Azimuth

Azimuth (and Bearing)

Altitude

At the North PoleAt the EquatorAt Northern mid-latitudes How the sky appears to move

At Northern mid-latitudes

Apparent Motion of The Celestial Sphere Some constellations around the Celestial North Pole never set. These are called “circumpolar”. The circle on the celestial sphere containing the circumpolar constellations is called the “circumpolar circle”.

Apparent Motion of The Celestial Sphere Circumpolar Stars

The View From Northern Mid-Latitudes

Apparent Motion of The Celestial Sphere (2)

North vs. South

At the North PoleAt the EquatorAt Northern mid-latitudes How the sky appears to move Latitude and Altitude of Polaris Latitude at North Pole = 90° N Altitude of Polaris = 90° Latitude at Equator = 0° Altitude of Polaris = 0° Latitude here = 34° N Altitude of Polaris = 34°

The Celestial Sphere (3)

So, why is it hotter in the summer?

WRONG ANSWER Number 1 INCORRECT answer. (Bite your tongue before saying this.) “We are closer to the Sun in summer.” We do NOT orbit the Sun like this, coming in closer during summer!!

NOT DUE TO DISTANCE Earth’s distance from the sun has only a very minor influence on seasonal temperature variations. Sun Earth in July Earth in January Earth’s orbit (eccentricity greatly exaggerated)

The Seasons Earth’s axis of rotation is inclined vs. the normal to its orbital plane by 23.5°, which causes the seasons.

Why is it warmer in summer? Two reasons 1.More direct sunlight 2.Longer days

More Direct Sunlight

Tilt Gives Us Seasons

Longer Days in Summer

The Seasons The Seasons are only caused by a varying angle of incidence of the sun’s rays. We receive more energy from the sun when it is shining onto the Earth’s surface under a steeper angle of incidence.

Obliquity of the Planets

Important Points in the Orbit

The Motion of the Planets The planets are orbiting the sun almost exactly in the plane of the Ecliptic. Jupiter Mars Earth Venus Mercury Saturn The Moon is orbiting Earth in almost the same plane (Ecliptic).

When should I go look for a planet? Inferior Planets – closer to the Sun than Earth (pictured here) Superior Planets – farther from the Sun than Earth When it is at GREATEST (MAXIMUM) ELONGATION. Elongation is the angle between the Sun and the planet from our point of view. A

The Inferior Planets P4 = Greatest Eastern Elongation P2 = Greatest Western Elongation Sunrise Sunset

Inferior Planets

Can you see it NOT at max. elongation?

Superior Planet Maximum Elongation = 180º