Unit 1Movement of the Earth, Seasons, Coordinates and Observing the night sky

Stars seem to be on the inner surface of a sphere surrounding the Earth.

Celestial Sphere

Stars that appear close in the sky may not actually be close in space.

Celestial Sphere

Celestial Poles are points on the celestial sphere north and south of the Earth’s north and south poles.

Celestial Sphere

Imaginary circle on the celestial sphere directly above the Earth’s equator.

Celestial Equator

Celestial Coordinates• Horizon – Edge of our

local sky– We can only see half of the

celestial sphere at one time.

• Zenith – The point straight overhead on the celestial sphere.– What you see if you lie on

your back and look straight up

• Altitude – How many degrees above the horizon• Azimuth – How many degrees along the horizon from the north

Celestial Coordinates

• Declination (Dec)• Degrees north or

south of celestial equator

• Right ascension (Ra)• Measured in hours,

minutes, and seconds eastward from position of the Sun at vernal equinox

Celestial Coordinates

Celestial CoordinatesCompare and contrast Ra, Dec, Latitude and Longitude.

Ecliptic• The apparent path

of the sun on the celestial sphere over the course of a year.

• Tilted at an angle of 23.5° with respect to the celestial equator

The 12 constellations the Sun moves through during the year are called the zodiac

Location of all planets when they are visible in the sky.

Ecliptic

Diurnal MotionApparent rising andsetting of stars

• Revolution – One body moves around another

• Rotation– Movement of an object around its axis

Earth’s Orbital Motion

• Solar Day – Our observed daily cycle (Noon to Noon)

• Sidereal Day – Time it takes for a star to be in the exact same position in the sky. (23.93 hours)

Earth’s Orbital Motion

Summer Solstice is June 21 – Sun at it’s northernmost point (above celestial equator) Winter Solstice is December 21 – Sun at it’s southernmost pointThis is caused by the tilt of the earth on it’s axis.

Responsible for seasons (not distance from Sun)

Points where path of the sun crosses celestial equator are vernal (March 21) and autumnal (Sept 21) equinoxes. Tropical Year = 365 solar days

Interval of time from one equinox to the next

Earth’s Orbital Motion

Precession: Rotation of Earth’s axis itselfmakes one complete circle in about 26,000 years

Earth’s Orbital Motion

Earth’s Orbital Motion

A sidereal year is the time it takes for Earth to orbit once around the sun relative to fixed stars.Tropical year follows seasons (365 days)

Sidereal year follows constellations (365.256363004 days)In 13,000 years July and August will still be summer, but Orion will be a summer constellation.

Why we have leap year!

Earth’s Orbital Motion

Moon Phases

Phases are due to different amounts of sunlit portion being visible from Earth.

Moon Phases

Synodic Month – Time it takes the moon to go through a whole cycle of phases (29.5 days).

The Motion of the MoonSidereal Month - Time to make full 360°revolution around Earth (about 2 days shorter than synodic month).

Tides are due to gravitational force on Earth from the Moon The force on near side of Earth is greater than the force on far side. Water can flow freely in response.

The Tides

The Sun has less effect, but it does modify the lunar tides.Spring tides- more tidal differentiationNeap tides – less tidal differentiation

The Tides

Tides tend to exert a “drag” force on Earth, slowing its rotation.This will continue until Earth rotates synchronously with the Moon, so that the same side of Earth always points toward the Moon.

The Tides

This has already happened with the Moon, whose near side is always toward Earth.

The Tides

Occurs when Earth is between the Moon and Sun

• Partial when only part of the Moon is in shadow (Penumbra)

• Total when all is in shadow (Umbra)

Lunar Eclipse

Lunar Eclipse

Occurs when the Moon is between Earth and Sun

Solar Eclipse

Solar Eclipse

Annular Solar Eclipse

Moon is too far from Earth for total.

Eclipses don’t occur every month because Earth’s and the Moon’s orbits are not in the same plane.

Solar Eclipse Tracks, 2010 - 2030

Measure baseline and angles, and you can calculate distance.

Tan = opp / adj

Tan B = Distance to Object / baseline

Distance to Object = (Tan B)(Baseline)

Triangulation

Similar to triangulation, but looking at apparent motion of object against distant background from two vantage points

Parallax

The greater the parallax, the closer the star

Parallax

Angular Measure

The angular diameter is proportional to the actual diameter divided by its distance. If any two of these quantities are known, the third can be determined.

The basic unit of angular measure is the degree (°).

Astronomers use angular measure to describe the apparent size ofa celestial object

Angular Measure

Stars can be imagined to be on inside of celestial sphere; useful for describing location.

Plane of Earth’s orbit around Sun is ecliptic; at 23.5° to celestial equator.

Angle of Earth’s axis causes seasons. Moon shines by reflected light, has phases. Solar day ≠ sidereal day, due to Earth’s rotation

around Sun. Synodic month ≠ sidereal month, also due to

Earth’s rotation around Sun

Review

Tropical year ≠ sidereal year, due to precession of Earth’s axis

Distances can be measured through triangulation and parallax.

Eclipses of Sun and Moon occur due to alignment; only occur occasionally as orbits are not in same plane.

Apparent size can be determined using angular measurement

Review