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Astronomy sphere Meridian poles keplers and newton laws

In this lesson we discusse Astronomy sphere Meridian poles keplers and newton laws

celestial sphere

the apparent surface of the imaginary sphere on which celestial bodies appear to be projected. north celestial pole, star trails, zenith, meridian, celestial equator, RA and DEC


an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator

north celestial pole

The location on the celestial sphere directly above the Earth’s northern rotation pole.


the brightest star in the night sky

astronomical unit

a unit of length used for distances within the solar system

light year

the distance that light travels in a vacuum in 1 year


the study of all matter and
energy in the universe, its structure,
organization and evolution

lunar eclipse

the blocking of sunlight to the moon that occurs when Earth is directly between the sun and the moon

solar eclipse

occurs when the Moon passes directly between the Sun and Earth and casts a shadow over part of Earth


point in the sky directly overhead; highest point


the apparent path of the sun through the stars


from the Greek for “wanderers”
because these bright “stars” moved with respect to the rest (real stars) on timescales observable by the individual


has 360 degrees because of
Babylonians estimated 1 yr is about 360 days (this over 3000 years ago)


a circular diagram representing the 12 zodiacal constellations and showing their signs. -planets move, but stay within about 18 degrees of the ecliptic. Root word is same as Greek for animals (zoo)


A group of stars that form a pattern in the sky. constructed for mnemonic aid.

solar day

the period of time from which the sun is directly overhead until the next time the sun is directly overhead. It is 24 hours.

sidereal day

the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day. 23 hr 56m

Heliacal risings

The heliacal rising of a star (or other body such as the moon, a planet or a constellation occurs when it first becomes visible above the eastern horizon for a brief moment just before sunrise, after a period of time when it had not been visible

Kepler’s 1st law

the orbits of the planets are ellipses with the sun at one foci (nearly circular)

Kepler’s 2nd law

the line between a planet and the sun sweeps out equal areas in
equal times

Kepler’s 3rd law

square of orbital period increases with cube of semimajor axis (planets farther from the sun move more slowly)

conic section

(geometry) a curve generated by the intersection of a plane and a circular cone

Newton’s 1st law

in the absence of force, objects move in a straight line.

Newton’s 2nd law

force equals mass times acceleration—the definition of force. The 2nd most famous equation in history: F = ma

Newton’s 3rd law

action equals reaction, or force equals force.

Newton’s “4th Law”

• Gravity is universal
• Gravity cannot extend undiluted forever, after all, other planets orbit the Sun not the Earth
• This leads to Newton’s Inverse Square Law

Scale of Planet Volumes

• Jupiter would fit 1000 times into the volume of the Sun
• Earth would fit 1000 times into the volume of Jupiter
• Earth would fit 1 million times into the volume of the Sun

retrograde motion

actual or apparent motion of a body in a direction opposite to that of the (direct) motions of most members of the solar system or of other astronomical systems with a preferred direction of motion. As viewed from a position in space north of the solar system (from some great distance above the Earth’s North Pole), all the major planets revolve counterclockwise around the Sun, and all but Venus and Uranus rotate counterclockwise on their own axes; these two, therefore, have retrograde rotation.

declination (DEC)

the angular distance of a body north or south of the celestial equator. Declination and right ascension, an east-west coordinate, together define the position of an object in the sky. North declination is considered positive and south, ne

What is astronomy?

the first science. It is a study of all matter and energy in the universe, it’s structure, organization and evolution

How many earth’s can you fit in the sun?

1 million.

If earth is an orange; moon is a grape at the back of the room; what is the sun’s size and location?

It is about the size and location of the top of a nearby tree, a block away.

If Sun is an orange; earth is a pencil dot at arm’s length, where is the nearest star?

In Chicago.

If a galaxy is the size of 1/4 a paper, where is the nearest galaxy? And…where would be the edge of the universe?

The nearest galaxy is another page in the hand of the person next to you. The edge of the universe is at about the Austin city limits.

How many stars are in a galaxy?

200,000,000,000 (200 billion stars). Each star is unresolvable.

What was the motivation for astronomy? Why is astronomy so old?

The motivation is prediction, dating, navigation, harvest, plantings, births, deaths

When we look at the sun, we see it as it was how long ago?

8 minutes ago.

When we look at the moon, we see it as it was how long ago?

1.5 seconds ago.

When we look at the Alpha Centauri, we see it as it was how long ago?

4+ years ago.

What is the Saros cycle?

18 years and 11 days.

When does a eclipse of the sun occur?

When the moon passes in front of the sun. Not always a total eclipse.

What is a lunar eclipse?

When the moon passes in the shadow of the earth.

Can you see a lunar eclipse at the same time from anywhere on the earth where you can see the moon?

Yes. The timing is the same for all observers who can see it (on the moon side, in the night sky).

Can you see a solar eclipse at the same time from anywhere on the earth?

No. Only lasts for a brief time in a narrow range. It has a specific path.

What is a solar eclipse?

When the moon passes in front of the sun.

heliacal rising

The first day a star can be seen before sunrise. Occurs at the same time each year.