Astronomy: Time and the Shape of the Earth

• Astronomy is the study of celestial objects like stars and planets.

Time Measurement

• Early civilizations determined the length of a year by observing cyclical Earth events like seasons and star patterns.
• A leap year is needed because a year is approximately 365.25 days long.
• An extra day is added to February every four years to account for the extra .25 days each year.
• Jewish years follow lunar months, totaling 354 days.
• An additional month (Adar II) occurs every two to three years (7 times in 19 years) in the Jewish calendar to compensate for the difference between the lunar and solar years.
• Years are the time it takes Earth to orbit the sun
• Months were originally based on the time it takes the moon to orbit Earth.
• Weeks are based on the Torah.
• Days are the time it takes Earth to rotate once on its axis.
• Gravity is the force of attraction between masses.

Evidence of Earth's Round Shape

• Ships disappearing over the horizon proves the Earth is round.
• Changing star patterns in different hemispheres is evidence that the Earth is round.
• The Earth's curved shadow on the moon during an eclipse proves that the Earth is round.

Telescopes

• Optical telescopes use visible light to observe the universe.
• Non-optical telescopes use other forms of radiation (radio waves, microwaves) to observe the universe.
• A light year is the distance light travels in one year.
• The speed of light is just under 300,000 km/s.

Solar System Planets

• Inner/terrestrial planets: Mercury, Venus, Earth, and Mars
• Outer/gaseous planets: Jupiter, Saturn, Uranus, and Neptune

Constellations and Stars

• A constellation is a group of stars forming a pattern, with 88 constellations recognized.
• Ursa Major and Orion are examples of constellations.
• Apparent magnitude is a star's brightness as seen from Earth.
• An asterism is a section of a constellation, such as the Big Dipper (part of Ursa Major).
• Polaris, the North Star, always points north as it aligns with Earth’s axis and the North Pole.

Earth, Moon, and Orbits

• Earth, the Moon, and most planets have elliptical orbits
• Earth's seasons are caused by its 23° tilt.
• The side of Earth tilted towards the sun experiences summer.
• Tides are caused by the gravity of the moon and sun pulling on Earth’s bodies of water.
• A lunar eclipse occurs when Earth is between the sun and moon.
• A solar eclipse occurs when the moon is between the Earth and the sun.
• The same side of the moon always faces Earth because its rotation and orbit take the same amount of time.

Models and Celestial Objects

• Heliocentric model: Sun is at the center of the solar system.
• Geocentric model: Earth is at the center of the solar system.
• A dwarf planet is smaller than a planet and has not cleared its orbit; Pluto is an example.
• An astronomical unit (AU) is the average distance between Earth and the sun (150,000,000 km).

Kuiper Belt and Oort Cloud

• The Kuiper Belt is a region beyond Neptune containing dwarf planets, comets, and dust.
• The Oort Cloud is at the outer limit of the Solar System and is believed to be mostly ice comets.

Comets and Asteroids

• A comet is a small object made of dust, ice, and rock from the outer Solar System, and has an eccentric orbit and a tail.
• An asteroid is a rocky piece of space debris orbiting the sun mostly found in the Asteroid Belt between Mars and Jupiter.
• Meteoroid: Rocks floating in space broken off from other larger bodies
• Meteor: A meteoroid burning up in Earth's atmosphere
• Meteorite: A meteor that landed on Earth

Space Exploration History

• World War II ushered in the space age through the development of strong rockets and flight technology.
• Sputnik was the first satellite, launched in 1957.
• Yuri Gagarin from Russia (USSR) was the first man in space.
• Neil Armstrong from the USA was the first man on the moon.
• A space shuttle is a reusable airplane-like spacecraft developed by NASA.
• The Soyuz spacecraft is important for being the safest, most cost-effective, and most reliable spacecraft.
• Western space travelers are called Astronauts.
• Russian space travelers are called Cosmonauts.
• Chinese space travelers are called Taikonauts.

Ethical and Future Considerations

• Space travel raises ethical issues such as: responsibility for cleaning up space junk, space ownership, and the value of space travel relative to other investments.
• New technologies to aid space travel include warp drive, 3D printers using space dust, reusable rockets, space elevators, and cryosleep.
• Terraformation is the process of making non-livable planets habitable.

Space mysteries

• The sun is made of hydrogen gas, and its energy is produced through nuclear fusion, where hydrogen atoms fuse into helium.
• Aurora Borealis is a light phenomenon near Earth’s poles caused by charged particles from the sun interacting with Earth's atmosphere.
• Earth is protected from space radiation by the ozone layer and its magnetic field.

Stars and Galaxies

• The composition of stars can be determined using spectroscopy, which analyzes the wavelengths of light emitted by stars.
• At the end of its life cycle, a star can become a supernova, black hole, or white dwarf.
• A galaxy is a collection of stars, planets, and nebulae held together by gravity.
• Only 4% of the universe is visible matter and energy, with the remaining 96% composed of dark matter and dark energy.
• The universe is currently estimated to be 93 billion light years wide.