The Moon part deux

• Goal: To understand the interactions between the earth and the moon.

• Objectives:

1) To understand Eclipses – solar and lunar

2) To learn about what causes theTides

What path does the moon make in the sky?

• A) It makes a circle along the ecliptic plane.

• B) It makes a circle along the celestial equator.

• C) It does not make a circle.

• D) None of these.

The Moon’s orbit

• The moon’s orbit is offset from the ecliptic by 5 degrees.

• So, most of the time when it is in front of or behind the earth, it does not fall directly in line with the sun (for example in the new moon it would pass directly above or below the sun).

• There are 2 places in the orbit called the “nodes” where the plane of the moon’s orbit intersect with the ecliptic plane.

• When these intersect at either the new moon or full moon….

Lunar Eclipse

• At the full moon, if it lines up onto the node, will be eclipsed.

• This happens twice per year (with less than half of the earth being able to witness it – which means you can witness about 1 lunar eclipse every 1-2 years on average).

• There are 3 types of lunar eclipses.

•This why the moon turns red during a total lunar eclipse.

•Next lunar eclipses viewable from N. America will be 6 AM, October 8th, 2014 (yes that is a week from tomorrow!), April 4, 2015, and Sept 28, 2015

Solar Eclipse• Similarly, the moon can block the sun when it lines up

correctly in the new moon.

• However, since the moon is much smaller than the earth, the shadow is much smaller, and only a small part of the earth is cast in the moon’s shadow.

• While there are still 2 eclipses per year, the main effect is limited to smaller areas, so to view them from a specific city is far more rare.

• Next solar eclipse viewable from the lower 48 U.S. states will be August 21, 2017 (might want to head to Kentucky that day)

• There are also 3 types of solar eclipses.

• Partial eclipse:

The moon only covers up part of the sun.

• Annular eclipse:

Since the moon’s orbit is ELLIPTICAL, in some portions of the orbit it will appear smaller.

If the angular size of the moon is less than the sun, it will create a dark circle on the sun, but NOT entirely cover up the sun.

Annular Eclipse vs Total Eclipse

Tides:

• What are tides and how are they formed?

Tidal forces

• Tidal forces are the result of a net difference in the gravitational force on 2 sides of an object.

• So, the part of the earth facing the moon feels a greater tug than the part away from it.

• This creates a net force for the parts of the earth near to and far away (near to towards the moon, and far away is away from the moon).

• At the parts of the earth in the middle, there is no net force.

• So, material (water mostly) goes towards the two points of greatest force. Thus the high tide!

Basic tidal facts

• For most places on the earth, there are 2 high tides and 2 low tides a day.

• Since the earth rotates the position of the tides is about 10 degrees in front of the moon, and on the opposite side of the earth from that.

• Due to local coastlines, the sizes of the tides vary, but average to be about 1-2 m in height.

• The moon also affects land! The land tide is about 6 inches, but is so gradual we don’t feel it.

• Finally, since the moon orbits the earth, the time of the tides changes by about 50 minutes each day – the same as the rise/set times of the moon.

Tidally locked!

• The moon is tidally locked. That means that the position of the “high tide” for the ground on the moon is locked roughly into place.

http://csep10.phys.utk.edu/astr161/lect/time/tides.html

Tide from

the sun is

10% that

of the

moon, but

enough to

affect the

Magnitude

of the

tides.

Yearly variations:

• The Moon’s orbit is ELLIPTICAL!• So, at some points it is closer (closer = stronger

tides), and some further away.• The phase it is closest and furthest depends on

time of year.• So, at some times of the year you will have

stronger high tides, and some weaker.• Strongest tides occur when moon is closest to

the earth at either the full moon or new moon.

But wait, there’s MORE!

• Since the bulge is pulled forward (due to tidal friction), that bulge will tug on the moon, and the moon on the bulge.

• This will result in a force which slows down the earth’s rotation ( our days increase by 2 milliseconds per day per century).

• Since energy must be conserved, the moon gains orbital energy, and gets 4 cm further from the earth each year!

How do we know?

• Sorry hoax enthusiasts, but we know this by shining laser light onto mirrors we placed on the moon (i.e. we had to be there to do this, sorry). By finding the time it takes to come back, they can measure the distance to the moon accurate to a few millimeters.

Bizarre fact of day

• Billions of years ago, the earth’s rotational period would have been 18-20 hours, and the moon would have been 10X closer than it is now.

Conclusion

• There are 3 types of lunar and solar eclipses.• There are 2 of each eclipse each year, but much

less that we here in Indianapolis get to see.• Tides happen twice a day due to changes in

gravity on opposite sides of the earth.• The strength of high and low tides change based

on the elongation of the moon and the moon’s distance to the earth.

• These tides change the moon’s orbit, and the earth’s day.