Universe 1

Astronomy 20 1

The Tidal ForceThe Cosmic stretching force.

Astronomy 202

Tides

Ocean Tides Usually two high tides, two low tides per day. Tidal range about 6 feet in California. High tides 1 hour later each day—Moon

connection.

Alma Inlet, Bay of Fundy

Astronomy 203

Tides

Bay of Fundy

Highest tides in world - over 50 feet!

Bay of Fundy

Universe 2

Astronomy 204

Tides

Orbital Motion Reprised

bigger R → smaller v

R

Astronomy 205

Tides

Space Station vcirc is greater at "bottom" of station; less at "top." Objects in the station drift to top and bottom. Tidal force = stretching force Force ∝ 1/r3

Astronomy 206

Tides

Space Station

You are orbiting the Earth in theSpace Station. Watching for a fewminutes, you notice that objects inthe Space Station:

1) drift toward the center of theSpace Station.2) drift toward the walls of the SpaceStation.3) just float around, not drifting in anyparticular direction.4) fall to the floor.

Universe 3

Astronomy 207

Tides

The Tidal Force

The Tidal Force is a stretching force.

Astronomy 208

Tides

The Tidal Force

It depends strongly on distance.

Astronomy 209

Tides

Tidal ForceApplications

1. Ocean tides2. Tidal braking3. Rings

Universe 4

Astronomy 2010

Tides

Ocean Tides

Two bulges Two high tides, two low tides a day.

Astronomy 2011

Tides

Sun Tides

The Sun also exerts a tidal force on the oceans.Given that the Sun is 400 times farther awaythan the Moon, one would predict that:

1. The Sun tides are greater than the Moon tides.2. The Sun tides and the Moon tides are about the

same.3. The Moon tides are greater than the Sun tides.

Astronomy 2012

Tides

Body Tides

Bulges are created in solids, too—body tides.

Earth Moon

Universe 5

Astronomy 2013

Tides

Tidal Braking

Early on, fast rotatingMoon.

Tidal force slows rotation. Result: synchronous

rotation.

Astronomy 2014

Tides

Tidal Braking

Tidal braking slows Earths rotation Day is 1 sec longer in 50,000 years.

Astronomy 2015

Tides

The Receding Moon

Moon getting 4 cm farther away every year. 4.5 Gyr ago, the Moon was only 1/10 as far—

Mile-high tides! Future: no more total solar eclipses. :(

Universe 6

Astronomy 2016

Tides

4 Billion Years Ago

Astronomy 2017

Tides

Check Question

The Moon will recede from theEarth…

1) Forever2) Until the Earth’s rotation period

equals a month3) Until the Earth stops rotating

relative to the stars

Astronomy 2018

Tides

Rings

Close to a planet, tidal force preventschunks from forming a moon.

Roche limit = 2.5 radii Inside Roche limit: rings Outside Roche limit: moons.

Dione, Tethys, & Saturn's rings

Universe 7

Astronomy 2019

Tides

Check Question

A moonlet breaks 10,000 milesfrom the center of a planet with aradius of 5000 miles. The moonletwill…

1) Pull itself back together.2) Spread out into a ring.3) Fall into the planet.

Astronomy 2020

Tides

The Tidal Force