#### Our Universe

Welcome to the universe big questions posed and answered section.

• Is the Earth flat? How big is the Earth? Where are we?
• How far away is the moon and the sun? How massive are the Earth, the moon and the sun?
• How far away are the stars? What is the Milky Way. How far does it stretch?
• How big is the universe? Did it have a beginning? How is it changing? What is its composition?

The following describes how the first two sets of questions were answered.

#### Earth Shape

That the earth is a sphere was first established by the ancient Greek philosophers. With ships disappearing over the horizon gradually until only the mast could be seen and the Earth casting a disc shaped shadow during lunar eclipses settled the issue.

#### Earth Circumference

Eratosthenes (b 276 B.C.) measured the size of the Earth by measuring the shadowcast by stick at Alexandria (7.2deg) compared to ovehead sun at Syene a distance of 5,000 stades away. So this angle represents 7.2/360 or 1/50 of the total circumference of the Earth i.e. 250,000 stades. An Egyptian stade 157m gives c= 39250 Km (2% accurate).

Once the size of the Earth had been estimated, the sizes and distances to the sun and moon could be found, the groundwork already having been figured out by other philosphers.

#### Lunar Diameter

Assuming the sun is much further away than the Earth-Moon duo, the Earth's shadow is the size of the Earth's disc diameter. During a total lunar eclipse the time for the moon to become totally submerged in the Earth's shadow is about 50 minutes. The time for the moon to cross the entire shadow, which is an indication of the Earth's diameter is around 200 minutes. So the Earth's diameter is about 4 times the moons. With earth circumeference about 40,000 km, the Earth's diameter is 40,000/pi i.e.12,700km. So the moon's diameter is 1/4x12,700 or 3,200km.

#### Earth-Moon Distance

Ths distance to the moon can be found by similar triangles. If you just block out the Moon with your fingertip at arm's length, the ratio of fingernail width to arm length being about 100, so the distance to the Moon is 100 times its diameter i.e. 320,000km.

#### Distance to the Sun

Aristarchus suggested if moonshine is reflected sunlight, then halfmoon must occur when sun-Moon-Earth form a right angle triangle. He attempted to measure the angle between the Earth-Sun and Earth Moon. He got 87 degrees and deduced the Sun to be 20 times further away than the moon. In fact the angle is 89.85 degrees and so the Sun is 400 times further away. Good start though, a valid technique.

#### The Solar System

The ancient astronomers stood on firm ground whilst heavenly bodies moved relative to a static Earth. They developed a view where the Earth was fixed and the universe revolved around it. Philolaus of Croton (5th century BC) suggested the Earth orbited the Sun. Aristarchus (b. 310 BC) added to this stating the Earth spins on its axis every 24hrs. The ancient Greeks didn't agree and preferred the earth centred model (no constant wind, Earth gravity dominant and no shift of star positions).

#### Solar System Models

Astronomers attempted to fit their understandings, beliefs and observations to form Solar System (universe) models:

1) Ptolemy Earth centred model AD 150.

2) Copernicus Sun centred, circular planetry orbits (1540s)

Other figures of note:  Tycho Brahe (1588) suggested a mixture of (1) and (2). Kepler introduced elliptical orbits. Galileo's (c1610) observations made the sun-centred model compelling and Newton of course with his law's of motion and gravitation.

#### To come

Beyond the stellar distances and distances to nebulae menu options:

• Milky Way limited universe, nebulae, cosmic distance scale.
• Gravity (G) one of four fundamental forces in nature.
• How the value of G was first measured.
• How old is the Earth? How it's measured.
• How old is the universe?
• Steady state universe or Big Bang?
• Galileo's view of relativity and Einstein's Special Relativity.
• Newton and Einstein's views on gravity.
• Einstein's theory of general reativity.

For those wanting to know what speed we need to attain to orbit or escape Earth's gravity, click the 'Out Of Sight' menu option.