Astronomy Midterm 1 Review

Midterm 1 Review Overview

• Lockdown browser is required and needs to be working before the exam.
• Contact the professor or TAs for any related issues.
• iPads are allowed to be used in the exam.
• Calculators are permitted, but there will be mostly conceptual questions.
• There will be no formula sheet.
• Laptops/devices should be charged, and chargers should be brought.
• A UMD ID will be needed for the exam.
• It is not a take-home exam.

Contents of this Review

• Dimensional analysis will be covered.
• Distances and the scale of the universe will be analyzed.
• Sky motions and constellations will be discussed.
• Seasons, moon phases, and eclipses will be looked at.
• The geocentric and heliocentric models will be compared.
• Newton's and Kepler's Laws and planetary motion will be studied.
• A section on the Solar System will be included.
• Planets and their moons, which include terrestrial planets, jovian planets, and moons, will be discussed.
• Properties of light will be covered.
• Small Bodies will be examined.

Dimensional Analysis

• Dimensional analysis can work out astronomical problems and convert units.
• Dimensional analysis expresses measured quantities in terms of fundamental units.

Units of Distance

• AU (Astronomical Unit) is the distance from Earth to the Sun.
• AUs are used to measure small distances in the Solar System.
• 1 AU is 1.496 x 10^11 meters.
• Light-years measure the distance light travels in one year.
• Light-years are used to measure large distances between galaxies and nearby stars.
• 1 light-year is 9.461x10^15 meters.
• Parsecs measure the apparent angular distance of objects based on parallax.
• Parsecs are used to measure distances further than light years.
• 1 parsec is 3.086 x 10^16 meters.
• Meters are typical units of distance used in science.
• Kilometers equal 1000 meters and ‘Kilo' = 1k in standard SI units.

Distances in the Universe

• The solar system extends from 30-50 AU, encompassing planets, the asteroid belt, and the Kuiper belt.
• The Oort cloud is situated between 5,000 and 10,000 AU.
• The Milky Way is 26,000 light-years across on its Orion Arm.
• The Local Group spans 5 million light-years.
• The Local Supercluster (Virgo Cluster) extends 65 million light years.

Sky Motions - The Celestial Sphere

• The Celestial Sphere consists of two coordinate systems: Alt/Az and RA/Dec.
• The Altitude in the Alt/Az system is measured in degrees from -90 to 90, indicating height above the horizon.
• Azimuth measures the direction along the horizon, measured in degrees East from North (0 - 360 deg).
• RA (Right Ascension) in RA/Dec is the time from object west to vernal equinox and corresponds to Earth-longitude.
• Declination (Dec) measures degrees north of the equator (-90, 90) and corresponds to Earth-latitude.

The Celestial Sphere Terminology

• Zenith: The point directly above your head, no matter where you are.
• Horizon: The dividing line between the Earth and the visible part of the sky.
• Meridian: An imaginary line going from North to South, cutting through the Zenith.
• The North Celestial Pole (NCP): The Northern point around which stars rotate.
• The South Celestial Pole (SCP): The Southern point around which stars rotate.
• Ecliptic: Path of the sun throughout the year (& planets) – changes.
• Galactic Plane: Plane where the band of the Milky Way is concentrated.
• Celestial Equator: The projection of Earth's equator that crosses the center of the celestial sphere.

Constellations and Circumpolar Stars

• Circumpolar stars are located on or near Earth's axis of rotation and appear to move less than other stars.
• Polaris is the circumpolar star.
• The altitude of Polaris is 90 degrees when at the North Pole
• The altitude of Polaris 0 is degrees at the Equator.
• The altitude of Polaris is 39 degrees in College Park.
• Over time, the axis of rotation changes with respect to background stars due to procession.
• Precession will cause the view of the stars, circumpolar start and the monthly constellations to change.
• The Sun completes a full rotation around the sky over a year due to Earth's motion.
• The "Zodiac" constellations are the constellations that the sun is in front of during those particular months.

The Seasons, Eclipses, Moon Phases

• The Earth has a 23 degree tilt with respect to its axis.
• The Earth has a NEARLY circular orbit, so the distance from the sun is roughly constant.
• Seasons are about indirect vs direct light.
• Solstice: When the sun is at its lowest or highest point in the sky.
• Solstice ecliptic changes based on what hemisphere is closer to the Sun at any given time.
• Equinox: When the ecliptic and the equator intersect at a point during the year, resulting in roughly even temperatures and day lengths.

Lunar Eclipses

• Lunar eclipses form when the Earth is directly between the Sun and Moon.
• Earth's shadow passes over the Moon during a lunar eclipse.
• The Moon appears a dark red during a lunar eclipse.

Solar Eclipses

• Solar eclipses happen when the Moon is directly between the Earth and Sun.
• Eclipses only cover a small area.
• Eclipses have many fantastical and religious beliefs associated with them.
• Total solar eclipses are rare due to their small surface area.
• The Moon must be at the right distance from Earth so it looks the same size as the Sun.
• Solar and Lunar eclipses happen when the Moon, Earth, and Sun are all in line.
• The orbit of the moon is tilted 5 degrees with respect to the Earth-Moon System.

Geocentric vs Heliocentric models

• Geocentric models state that everything in the Solar System and the Universe rotates around the Earth.
• Heliocentric models state that planets in the Solar System orbit around the Sun.
• Stellar Parallax supports the Heliocentric model.

Kepler’s and Newton’s Laws: The Laws of Planetary Motion

• Kepler's 1st law: Orbits are ellipses, not perfect circles with the Sun at one focus.
• Kepler's 2nd law: Orbits trace out areas in equal times (applies to binary orbits as well).
• Kepler's 3rd law: The square of the period P of the orbit is proportional to the cube of the semi-major axis R.
• Newton's 1st law: An object at rest remains at rest unless acted upon by an external force (law of inertia)
• Newton's 2nd law: Fnet = mass x acceleration
• Newton's 3rd law: For every action, there is an equal and opposite reaction.
• Newton’s Law of Universal Gravitation describes the force of gravity exerted between two bodies.
• If you're ON the Earth, the gravity exerted simply reduces to Fnet = ma.

What's in the Solar System?

• The Solar System consists of the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
• Other objects: moons, trojan asteroids, asteroid belt (Ceres), kuiper belt (including dwarf planets like Pluto), interplanetary gas and dust.
• Jupiter has rings (small debris), moons (including galilean moons): 95, and trojan asteroids.
• Saturn has rings and moons (146) - Titan.
• Uranus has rings and moons (28).
• Neptune has rings (minor), trojan asteroids, and moons (14).
• Most objects rotate and orbit the sun counterclockwise where rotation is spinning on axis.
• Larger objects have NEARLY circular orbits and orbit in the same plane.
• Years depend on orbital speed and is described by Kepler's Law.
• The two main types of planets are Terrestrial and Jovian.
• Terrestrial planets are smaller in mass and denser.
• Jovian planets are larger in mass and hydrogen-rich.

Formation: The Nebular Theory

• Nebula is a massive cloud of gas and dust that experiences gravity.
• Denser material in center (needs warmer temps to condense).
• The inner parts of nebular disks are limited to asteroids.
• Large planets have more material that was accreted, a hydrogen/helium abundance, and are less energetic.
• Frost line: past where hydrogen forms ice.
• “Cold’ materials can coalesce here.

Terrestrial Planets

• These include Mercury, Venus, Earth, and Mars.
• They have mainly rocky and dense surfaces.
• Terrestrial Planets have relatively thin atmospheres as compared to the Jovian Planets.
• Some have few moons, but Earth is an exception.
• They have molten metal cores that allow for geologic activity.
• They have evidence of impact craters, except for Earth, which has significant geologic activity.

Jovian Planets

• Jovian planets includes Jupiter, Saturn, Uranus, and Neptune.
• They have no solid surfaces until you get to the icy and rocky metal cores.
• High mass, low density profiles - gases that may be tightly packed and pressurized.
• Their atmospheres (90% of the planet) are mainly Hydrogen and Helium, which coincidentally, are the lightest and most abundant gases in the universe.
• Jovian planets have high winds and atmospheric circulation patterns - like Jupiter's Big Red Spot.

The Greenhouse Effect

• Atmospheres trap heat and radiation, which is important for the maintenance of life on Earth.
• As radiation enters the atmosphere from the Sun, there is some absorption and reflection.
• Atmospheres should catch the right amount of heat to maintain stable temperatures, not in the case of Venus, or Mars!
• Particles in the atmosphere absorb and scatter light.
• Earth's atmosphere absorbs most wavelengths of radiation.
• Scattering occurs when the photons hit particle in the atmosphere and the light is bent, turning the sky red at sunset.
• Venus and Mars have atmospheres that that are extremely thick and thin, respectively.

Small Bodies

• Moons: Io, Europa, Ganymede, Callisto are important Jupiter moons, Titan is a moon of Saturn.
• Most moons are like the Earth’s moon or are small asteroids.
• Asteriods are chunks of ice, metal, and rock left over from the formation of the Solar System.
• Kuiper belt and asteroid belt are sources of asteroids.
• Comets are "Dirty snowballs" with highly elliptical orbits around the Sun.
• As they approach the Sun, the cold dust starts to shed off of the comet, forming their tails.
• Most comets are objects knocked off orbit from the Oort Cloud.