Astronomy 1010 Planetary Astronomy Fall_2015 Day-17

Course Announcements Dark Sky nights – Mon. 10/5 & Wed. 10/7 starting at 7:30pm – at the Observatory. Exam-2 will be Friday, Oct. 9 SW-chapter 4 posted: due Mon. Oct. 5 SW-chapter 5 posted: due Fri. Oct. 9

Definitions & Terms -1 Light: The primary source of information in astronomy for all objects. When we say “light” we are talking about everything from gamma-rays to radio-waves. This does NOT include gravity waves.

 Tides can affect the solid part of Earth, too.  A gravitational pull can stretch and deform a solid body.  Results in friction, which generates heat.  Friction also opposes the rotation of Earth, causing Earth to very gradually slow its rotation.  Days lengthen by about seconds every century.

 Earth’s mass is large compared to the Moon’s, so tidal forces on the Moon are strong.  Moon’s rotation and orbital period are locked: tidal locking.  This means the Moon’s rotation period equals its orbital period.

 Because of tides, Earth is not a perfect sphere.  Earth’s leading edge creates an acceleration on the Moon in its orbit, resulting in a bigger orbit.  The lunar month increases by seconds per century.

 Gravitational objects create Roche limits within which the effects from their tidal forces can destabilize other objects.  Planets, stars, and galaxies can be affected.

 Galaxies can be distorted with tidal tails.  Moons of planets can have great internal activity because of tides, perhaps allowing for conditions for life.

 In order to leave a planet’s surface, an object must achieve a velocity greater than the planet’s escape velocity.  Therefore, Earth’s escape velocity is MATH TOOLS 4.3

 The net tidal force is the difference in force on the two sides of the planet.  The Moon’s tidal forces on Earth are 2.2 times as strong as the Sun’s tidal forces on Earth, resulting in greater lunar tides than solar tides. MATH TOOLS 4.4

 Almost all knowledge of the universe beyond Earth comes from light.  Light can tell us about objects in space: temperature, composition, speeds, and more.

 Light moves at 300,000 km/s in a vacuum.  First measured by Rømer when observing Jupiter’s moons.  Speed is slower in other materials, e.g., glass.

 Light is a wave of combined electricity and magnetism, called an electromagnetic wave.  Changing electric and magnetic fields create a self-sustaining electromagnetic wave.

 Wavelength ( ): length between crests.  Amplitude: height.  Frequency (f): number of waves that pass by each second.  Period (P): time to complete one cycle.

 A long wavelength means low frequency.  A short wavelength means high frequency.  The speed of light, c, is constant.

 Visible spectrum: small range of wavelengths that humans can see.  Red light = longest wavelength ( ~ 750 nm).  Violet light = shortest wavelength ( ~ 380 nm).  “Roy G. Biv.”

 Visible light is just one small part of the electromagnetic spectrum: Gamma rays X-rays UV Visible IR Microwave Radio increasing increasing f decreasing

Lecture Tutorial EM Spectrum: (pg. 47) Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group.

i_Clicker Question Light & Spectra: EM Spectrum Parameters Light & Spectra: Number Line: EM Spectrum: Option-2