Astronomy Unit 1 The celestial sphere and the seasons

Coordinate system There are 2 coordinate systems that can be used to locate things in the sky. 1.Horizon system: it is based on your location and the momentary position of the object with respect to the horizon. 2.The Equatorial system: it is based on an imaginary set of grid lines starting with the equator and the meridian. These objects will be in the same positions at the same time of year.

Horizon system: key terms Horizon system Used to locate a star/planet using altitude (distance above horizon) and azimuth (direction) from a specific location. Zenith the point directly above you in the sky Nadir The point directly below you Horizon where the ”land” or tree-line meets the sky Altitude from 0º on horizon to 90º at zenith Azimuth from 0º at N, 90º at E, 180º at S, 270º at W

Notice that the position of stars change over the course of a few minutes or hours because Earth is spinning (the yellow streaks are fairies and fireflies)

Horizon system Problems Stars change position over the course of the night because of rotation of Earth—stars rise and set like the sun 2. Stars occupy different positions in the sky at different points on the earth (N vs. S hemispheres)

Equatorial (Celestial) Coordinate System Universal system for all locations Declination: similar to altitude or lattitude, degrees north or south of the equator Right Ascension: similar to azimuth or longitude, direction east or west of the meridian. – Measured in hours, minutes, and seconds because the earth is spinning over time (think time zones) Based on the celestial sphere, an imaginary projection of a sphere surrounding us where all celestial objects are placed.

Celestial Sphere Imaginary sphere onto which all stars are projected (imaginary sphere of what we see) North Celestial Pole Point on celestial sphere directly above North Pole of earth Polaris The North Star because it is practically located on the North Celestial Pole You can read more about it in your textbook on pg. 20

Diurnal Circles from mid-latitude Circumpolar stars group of stars that do not set below horizon (they go around Polaris) Latitude and Polaris The angle of Polaris above your horizon is the same as your latitude in degrees.

Note that the stars are moving counter clockwise across the sky.

If you are standing at the North Pole, your view of the celestial sphere looks like this… Remember: the altitude of Polaris is the same as your latitude. Where would Polaris be if you were on the equator?

If you are standing at the equator, your view of the celestial sphere looks like this…

If you are standing in Atlanta, your view of the celestial sphere looks like this…

We can also measure objects on the sphere The apparent distance between two points on the dome of the sky is the angular distance, which is the angle between the two objects, measured with the vertex at the center of the sphere (earth). Any angle measured with the vertex at the center is said to be measured in degrees of arc. – Fractional values can be specified in arcminutes and arcseconds. – Note that 1 degree of arc is always the same size on the sphere of the sky, no matter where it is measured on the "surface" Angular distance or angular size can be measured in any direction on any part of the sky.

Measuring distance in the sky

Angular Measurements In astronomy, angular measurements are made to determine locations of celestial objects from their position Degree = a circle’s divided into 360 degrees See diagram at right to determine the approximate size of 1, 5, 10, and 15 degrees Moon and sun are only 0.5 ° (stays constant--if it looks bigger...it’s an illusion)

Defining a day. (These terms are not set times, just positions) Your position, depending on where the earth is in it’s orbit with respect to other planets in their orbit, will allow you to see the planets at night, or not.

Sidereal Day vs. Solar Day Sidereal Day The time it takes a star to line back up with Earth 23 hours, 56 minutes, 4 seconds (~4 minutes shorter than a 24 hr day) Therefore stars rise ~ 4 minutes earlier each night Mean Solar Day =24 hrs (avg. time for Sun to return to same position)

What is the ecliptic? Definition #1 the daily path of the sun in the sky Definition #2 the path of the sun on the celestial sphere over the course of the year

The path of the sun changes over the course of the year

Mercury Venus Mars Jupiter moon Motion and Positioning of the planets in the sky. You can only see the planets that are on the night side of the Earth

Notice that the planets lie along the ecliptic because the solar system is in the same plane (a flat disc) Imagine the sun here below horizon in west

Motion of the Planets The Sun, Moon and planets can always be found in a Zodiac Constellation on the ecliptic. Planets "wander" across the celestial sphere and through the stars in two ways: 1. Direct Motion - normal eastward movement of planets 2. Retrograde Motion - occasional westward movement of planets

The Motion of the Planets Mercury appears at most ~28° from the sun. It can occasionally be seen shortly after sunset in the west or before sunrise in the east. Venus appears at most ~46° from the sun. It can occasionally be seen for at most a few hours after sunset in the west or before sunrise in the east.

Planets that are NOT visible Planets may not be visible at night because of where they might be in their orbits. What if Jupiter were here?

Retrograde Motion Retrograde motion occurs when an inner planet passes by an outside planet and the outside planet appears to go “backwards” in the sky for a few weeks (this doesn’t happen in a night) Click for Animation The movement is apparent— Mars does NOT really move like that in space. The position of mars shifts against the backdrop of distant stars.

The positions of the stars change over the course of the year because the Earth orbits the Sun… Off-season constellations are “up” during the daytime Notice that between June and August, the Sun is directly “in” Gemini Notice that June to August, Sagittarius is prime viewing during summer nights

Phases of Inner Planets The inner planets go through phases similar to the Moon. Just as with the Moon, it is the positioning between the planet, the Earth, and the Sun that determines how much of the illuminated portion we see, and hence, the phase. Figure 4 shows the phases of Venus. One big difference between the phases of the inner planets and the phases of the Moon is that the angular size of the planet changes with the phase. As you can see from Figure 4, the full Venus appears much smaller than a crescent Venus because a crescent Venus is much closer to us. This means Venus is actually at its brightest when it is in a crescent phase.

Precession The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession. At left, gravity is pulling on a slanted top. Wobbling around the vertical.

Precession As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. It will be closest to Polaris ~ A.D There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, it will be close to Vega in the constellation Lyra. It moves 1 degree every 72 years

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

The Seasons They are not related to Earth’s distance from the sun. In fact, Earth is slightly closer to the sun in (northern-hemisphere) winter than in summer. Light from the sun The Seasons are a result of how direct the sunlight is that strikes Earth.

The more direct light is more intense and provides more energy or heat.

The Seasons Northern summer = more direct light, sun higher in sky, daytime sunlight is longer Northern winter = less direct light, sun lower in sky, daytime sunlight is shorter