Intro to Astronomy Midterm Study Guide

Astronomy

The scientific study of matter in outer space, such as the positions, dimensions, energy, and evolution of stars and planets

Rotation

An object spinning on an axis

Revolution

One object in orbit around another

Earth is a planet that ______ on its axis once per day

rotates

Earth orbits the Sun and completes one ______ per year

revolution

There are ____ planets in the solar system

8

There are ___ minor planets in the solar system

3

The ____ is one of hundreds of billions of stars in the Milky Way Galaxy

Sun

How old is the universe?

The universe is about 14 billion years old

Constellations

a pattern of stars randomly distributed

How many constellations are there?

88 constellations

How many zodiacs are there?

13 zodiacs

What do the constellations of the Zodiac have in common?

The constellations of the zodiac all lie along the ecliptic, which is the apparent path the Sun takes across the sky throughout the year.

How many degrees is the Earth tilted?

The earth is tilted 23 ½ degrees

Measures of angles

In astronomy, angles are commonly measured in degrees, arcminutes, and arcseconds to describe the apparent size and position of celestial objects.

Degrees (°): The most basic unit, where a full circle is 360°. A right angle is 90°.

Arcminutes (′): One arcminute is 1/60th of a degree.

Arcseconds (″): One arcsecond is 1/60th of an arcminute, or 1/3600th of a degree.

Celestial sphere

a map of where the different celestial bodies are located

North celestial pole

The point in the sky directly above Earth's North Pole

South celestial pole

the point above Earth's South Pole

The ecliptic

the location of the sun on the celectsial sphere throughout the year

Equinox

The time or date (twice each year) at which the sun crosses the celestial equator, when day and night are of approximately equal.

What are the two equinoxes

The vernal and autumnal equinoxes mark the two times each year when the Earth's equator is most directly aligned with the sun, resulting in approximately equal hours of daylight and darkness worldwide.

Vernal Equinox

The vernal equinox, also known as the spring equinox, occurs in March and signals the beginning of spring in the Northern Hemisphere (March 21).

Autumnal Equinox

The autumnal equinox, or fall equinox, occurs in September and marks the start of autumn in the Northern Hemisphere (September 21).

Solstice

The time or date (twice each year) at which the sun reaches its maximum or minimum declination, marked by the longest and shortest days (about June 21 and December 22)

Winter Solstice

The winter solstice is the shortest day of the year in the Northern Hemisphere, occurring when the Northern Hemisphere is tilted furthest away from the sun. It's also the longest night of the year (December 21st or 22nd).

Summer Solstice

The summer solstice is the longest day of the year in the Northern Hemisphere and occurs when the sun's apparent path is farthest north. It happens when the Earth's tilt is most inclined towards the sun (between June 20 and 22).

Equatorial Coordinate System

A way to locate objects in the sky using two coordinates: declination and right ascension

Declination

The coordinates vertical to the celestial sphere/the Earth's equator is the zero coordinate of the declination (-90 to 90)

The north celestial pole is Declination 90 degrees and the south celestial pole is declination of -90 degrees

Right ascension

Along the equator (the second coordinate)

Right ascension is measured in hours/ 24 hours degree of right ascension (0 to 24)

(Fixed for stars variable for sun, moon, planets)

Local Horizon aka alt-azimuth

The local horizon coordinate system, also known as the alt-azimuth system, uses the observer's local horizon as the fundamental plane for measuring the position of objects in the sky.

Two coordinates: altitude, which is the vertical angle above the horizon, and azimuth, which is the horizontal angle measured

Altitude

This coordinate in this local horizon coordinate system represents how the angle above the horizon an observer must look to see a celestial object. The horizon is at 0° altitude while the zenith is located at 90° altitude. (o to 90)

Azimuth

This coordinate, called the Azimuth angle, is measured in degrees around the horizon circle where 0° corresponds to due north. The circle extends all the way around the horizon, 90°being due east; 180°, due south; 270°, due west; and north, back to 0°.

Direction (N, S, E, W) or Azimuth (0 to 360)

Daily motion

The rotation of Earth is responsible for the daily motion of the sky and the time measurement day

What are the causes of moon phases?

The Moon does not emit light. It only reflects light. So, Moon phases are caused by the changing angles at which we view the sunlit portion of the Moon as it orbits the Earth

Sideral month

The time it takes for the Moon to orbit the Earth and return to the same position relative to the distant stars

Lunar month

The time it takes for the Moon to cycle through all its phases (new moon, first quarter, full moon, last quarter) and return to the same phase again

What are the phases of the moon?

The eight lunar phases are, in order: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent. The cycle repeats about once a month (every 29.5 days).

New moon

This is the invisible phase of the Moon, with the illuminated side of the Moon facing the Sun and the night side facing Earth. In this phase, the Moon is in the same part of the sky as the Sun and rises and sets with the Sun. (The Moon is between the Earth and the Sun)

Waxing crescent

This silver sliver of a Moon occurs when the illuminated half of the Moon faces mostly away from Earth, with only a tiny portion visible to us from our planet. It grows daily as the Moon's orbit carries the Moon's dayside farther into view. Every day, the Moon rises a little bit later.

First quarter

The Moon is now a quarter of the way through its monthly journey and you see half of its illuminated side. People may casually call this a half moon, but remember, that's not really what you're witnessing in the sky. You're seeing just a slice of the entire Moon ― half of the illuminated half.

Waxing gibbous

More than half of the Moon is lit, and the illuminated portion continues to grow.

Full moon

This is as close as we come to seeing the Sun's illumination of the entire day side of the Moon (so, technically, this would be the real half moon). The Moon is opposite the Sun, as viewed from Earth, revealing the Moon's dayside. A full moon rises around sunset and sets around sunrise. The Moon will appear full for a couple of days

Waning gibbous

As the Moon begins its journey back toward the Sun, the opposite side of the Moon now reflects the Moon's light. The lighted side appears to shrink, but the Moon's orbit is simply carrying it out of view from our perspective. The Moon rises later and later each night.

Last quarter

The Moon looks like it's half illuminated from the perspective of Earth, but really you're seeing half of the half of the Moon that's illuminated by the Sun ― or a quarter. A last quarter moon, also known as a third quarter moon, rises around midnight and sets around noon.

Waning crescent

The Moon is nearly back to the point in its orbit where its dayside directly faces the Sun, and all that we see from our perspective is a thin curve.

Rise and set times of the moon

New Moon: Rises and sets with the sun.

Full Moon: Rises at sunset and sets at sunrise.

Quarter Moons: Rise around midday and midnight, respectively, and set around midnight and midday.

Crescent and Gibbous Moons: Rise and set times vary, but generally, they follow a pattern related to the quarter moon times, with waxing (growing) phases rising earlier and waning (shrinking) phases rising later.

Moon's Path: The moon's path across the sky is similar to the sun's, but with some variations due to its orbit.

Eclipse

Occurs when the moon is on or near the ecliptic

Lunar eclipse

If the Earth is between the Sun and the Moon (the full Moon phase), Earth's shadow will cover the Moon. This is a lunar eclipse

Solar eclipse

If the Moon is on the ecliptic when it is in between the Sun and Earth (in the new Moon phase) it will block out the view of the Sun from Earth. This event is called a solar eclipse.

Types of lunar eclipses

Penumbral: The Moon passes through the penumbra, which is the faint outer part of Earth's shadow. The change in brightness is often subtle and may not be easily noticeable.

Partial: Part of the Moon enters the umbra, the dark inner shadow of the Earth. This results in a noticeable darkening or even a reddish hue on the portion of the Moon within the umbra.

Total: The entire Moon is engulfed by the umbra. During this phase, the Moon can take on a reddish or coppery color due to the way sunlight is bent and scattered by Earth's atmosphere as it passes through the Earth's atmosphere and onto the Moon.

Types of solar eclipse

Partial: A partial solar eclipse occurs when the Moon only blocks part of the Sun, creating a crescent shape.

Total: A total solar eclipse happens when the Moon completely covers the Sun.

Annular: An annular solar eclipse occurs when the Moon appears smaller than the Sun, leaving a bright ring of sunlight visible around the Moon.

Annual Motion

The apparent yearly movement of celestial objects, particularly the Sun, across the sky as seen from Earth

Sun on the ecliptic

The orientation of Earth's ecliptic plane with respect to the Sun, combined with the Earth's axial tilt, determines the varying distribution of sunlight throughout the year. This leads to the phenomenon of seasons, with the equinoxes and solstices marking the transition points between them.

Earth's revolution around the sun

Earth's revolution around the sun refers to its orbital path, a slightly elliptical orbit that takes approximately 365.25 days to complete. This is also known as Earth's year.

Seasons

The four seasons (spring, summer, autumn, and winter) are caused by the tilt of the Earth's axis as it orbits the sun. This tilt, combined with Earth's yearly revolution around the sun, results in different hemispheres receiving varying amounts of direct sunlight throughout the year, leading to the temperature and daylight hour variations we experience as seasons.

Spring

Cause: The Northern Hemisphere begins to tilt more directly towards the sun.

Characteristics: Temperatures gradually warm up.

Daylight hours increase.

Summer

Cause: The Northern Hemisphere is tilted most directly towards the sun, receiving the most direct sunlight.

Characteristics: Temperatures are at their warmest. Daylight hours are the longest.

Autumn (fall)

Cause: The Northern Hemisphere begins to tilt away from the sun.

Characteristics: Temperatures gradually cool down.

Daylight hours decrease.

Leaves change color and fall from trees.

Winter

Cause: The Northern Hemisphere is tilted furthest away from the sun, receiving the least direct sunlight.

Characteristics: Temperatures are at their coldest.

Daylight hours are the shortest.

The Scientific Process

The process of observation, explanation, testing, and revision

Ptolemy and the Geocentric Model

Claudius Ptolemy (90-168 A.C.E.) was a Greek mathematician, astronomer, geographer, astrologer, and music theorist

Ptolemy's geocentric model placed Earth at the center of the universe, with the Sun, Moon, and planets revolving around it in a complex system of circles called epicycles and deferents

Copernicus and the Heliocentric Model

Nicolaus Copernicus (1473 - 1543 A.C.E.), was a Polish astronomer and mathematician, known as the "father of modern astronomy"

The Copernican heliocentric model, developed by Nicolaus Copernicus, proposes that the sun, not the Earth, is at the center of the solar system

Tycho Brahe and Accurate Measurements

Tycho Brahe (1546-1601 A.C.E) is best known for his ability to make careful, precise measurements of celestial motions and then document those observations in great detail.

Tycho used a technique that relied on parallax. Parallax is the apparent shifting of position of an object relative to the background as the viewpoint of an observer changes from one position to another.

Kepler and the Three Laws of Planetary Motion

Johannes Kepler (1571-1630 A.C.E.) was a mathematician with a deep interest in astronomy.

Kepler's Three Laws of Planetary Motion:

1. Each planet orbits the Sun in the shape of an Ellipse and the Sun occupies one focus of the ellipse.

2. The imaginary line joining a planet and the Sun sweeps out equal areas of space during equal time intervals as the planet orbits.

3. The squares of the orbital periods of the planets are directly proportional to the cubes of the semi-major axes of their orbits.

Galileo and his contributions

Galileo Galilei (1564-1642 A.C.E.)

With the aid of his telescope, Galileo made four significant observational discoveries: Lunar features, including craters and mountains, Sunspots, The four largest natural satellites (moons) of Jupiter and the phases of Venus. Of course, Galileo could not have made his observational discoveries without the aid of a telescope. Although Galileo did not invent the telescope, he is generally credited with having been the first person to use the telescope for astronomical purposes

All of Galileo's discoveries helped support and prove the Heliocentric Model

Newton and his contributions to Gravity

Sir Isaac Newton (1642- 1727) was the first to develop a mathematical expression describing the relationship between the mass of two objects, the distance separating them and the magnitude of the gravitational force experienced by each of them. In 1687, he published this work, where he introduced the Law of Universal Gravitation.

The Law of Universal Gravitation states that every particle exerts an attractive force, F, on every other particle and that attractive force is proportional to the product of the masses of the objects and inversely proportional to the square of the distance between them

Put simply:

The more massive the objects are, the stronger the gravitational force they will exert on each other.

Also, the closer the objects are to one another, the stronger will be their gravitational pull on each other.

Gravity

The force that attracts a body toward the center of the earth, or toward any other physical body having mass.

Mass v Weight

Mass (kg):

Remains constant, independent of where they are located.

Weight (N or lb):

Defined as the force on a body due to gravitational interaction with another body.

An object of a given mass would have a certain weight on Earth and a different weight on the Moon

Orbit

The curved path of a celestial object or spacecraft around a star, planet, or moon, especially a periodic elliptical revolution.

Nebular Theory

The Sun and planets originated from a rotating cloud of gas and dust called a solar nebula. Over time, this nebula contracted under gravity, flattened into a disk, and eventually formed the Sun and the planets through processes like accretion (small particles collide and stick together).

Formation of the Solar System: Contracting, Spinning Nebula, Increased Central Heating

As the nebula contracted under its own gravity, it began spinning faster due to the conservation of angular momentum. This spinning caused the nebula to flatten into a disk shape. As material continued to fall inward, the density and pressure at the center increased. This resulted in rising temperatures in the center, eventually leading to nuclear fusion that ignited the Sun.

Formation of the Solar System: Composition of the Nebula

The nebula was primarily made of hydrogen and helium gas, which are the most abundant elements in the universe. It also contained trace amounts of heavier elements such as carbon, oxygen, silicon, and iron, which came from earlier generations of stars. These heavier elements played a crucial role in forming rocky planets and solid materials. The combination of gases and dust allowed for the creation of both gas giants and terrestrial planets.

Formation of the Solar System: Importance of Temperature/Location for Planet Formation

Temperature and distance from the Sun determined which materials could solidify in different parts of the disk. In the inner regions, where it was hotter, only metals and rocky materials could condense, leading to the formation of terrestrial planets like Earth and Mars. Farther out, where it was cooler, gases and ices could condense, allowing giant planets like Jupiter and Saturn to form. This temperature variation explains the different types of planets in the Solar System.

Formation of the Solar System: Accretion Process

Accretion is the process by which small particles and dust in the nebular disk stuck together to form larger bodies. These tiny grains collided and combined over time, forming planetesimals, which are the building blocks of planets. As planetesimals grew, their gravity pulled in more material, rapidly increasing their size. Over millions of years, this process led to the formation of full-sized planets and other celestial bodies.

Overall structure of the solar system

- Sun in the center

- 8 major planets

- Dwarf planets

- Asteroids

- Comets

- Meteoroid, Meteor, Meteorite

What are the 8 major planets?

In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

What are the two major planet types?

Terrestrial and Jovian

Terrestrial Planets

Terrestrials (Mercury, Venus, Earth, Mars)

- Solid Surfaces

- Small

- Rock/Metals, etc.

Cause and Consequences of Plate Tectonics and Volcanism

Cause of Plate Tectonics and Volcanism:

- Driven by internal heat from planet formation and radioactive decay.

- Convection currents in the mantle cause tectonic plates to move.

- Volcanism occurs when magma rises through cracks due to pressure or plate interactions.

Consequences of Plate Tectonics and Volcanism:

- Shape planetary surfaces, forming mountains, rift valleys, and basins.

- Cause earthquakes and volcanic eruptions that release gases into the atmosphere.

- Create new landforms and recycle nutrients, influencing climate and potential habitability.

- Play a major role in maintaining Earth's dynamic, life-supporting environment and shaping other terrestrial planets.

Impact craters

Impact craters are a significant geological feature on terrestrial planets, resulting from the collision of asteroids or comets with planetary surfaces

Mars and Evidence of Water

There is substantial evidence that liquid water once flowed on Mars and that it may still exist in some locations today. Scientists continue to investigate the extent and nature of water on Mars, seeking to understand its role in the planet's past and potential for supporting life.

Venus and the Greenhouse Effect

Venus has a runaway greenhouse effect due to its dense, carbon dioxide-rich atmosphere, making it far hotter than Earth. This extreme temperature is caused by the atmosphere trapping heat, a phenomenon exacerbated by the planet's proximity to the Sun and its thicker atmosphere.

Earth's Moon Composition and Origin

The most accepted theory for how the Moon formed is the giant-impact hypothesis. It suggests that about 4.5 billion years ago, a Mars-sized object called Theia crashed into early Earth. The impact sent debris from both Earth and Theia into space, which eventually came together to form the Moon. The Moon's makeup is similar to Earth's mantle, has a smaller iron core, and contains fewer volatile elements than Earth.

Jovian Planets

Jovians (Jupiter, Saturn, Uranus, Neptune)

- Large

- Gaseous

- Rings, etc.

- Many Natural Satellites

Natural Satellites

A natural satellite is a celestial body that orbits a planet or other larger object, and is not man-made. Ex: Moons

Important Satellites of Jupiter

Galilean Moons of Jupiter: Io, Europa, Ganymede, and Callisto are Jupiter's four largest satellites.

Io: Most volcanically active body in the Solar System, Surface covered in lava plains and volcanoes, Volcanic activity caused by tidal heating from Jupiter and nearby moons, Slightly larger than Earth's Moon, with a similar density.

Europa: Icy surface with a possible subsurface saltwater ocean containing more water than Earth's oceans, Ice shell may be several kilometers thick or as thin as 200 meters, Tidal forces cause flexing, possibly driving hydrothermal activity that could support life, Slightly smaller than Earth's Moon.

Ganymede: Largest moon in the Solar System, Shows signs of past geological activity and has its own magnetic field

Callisto: Outermost Galilean moon, Heavily cratered, showing little to no geological activity since its formation.

Important Satellite of Saturn

Saturn's moon Titan is noteworthy for having a thick atmosphere primarily composed of nitrogen, with significant amounts of methane. It is also unique as the only other place in the solar system besides Earth known to have liquids on its surface, although these liquids are hydrocarbons like methane and ethane, due to the extremely cold temperatures.

Dwarf Planets

Dwarf planets are celestial bodies that orbit the sun, are nearly round due to their own gravity, but have not cleared their orbital path of other objects

Pluto: Once considered the ninth planet, Pluto is now classified as a dwarf planet. It resides in the Kuiper Belt, a region beyond Neptune.

Ceres: Located in the asteroid belt between Mars and Jupiter, Ceres was once considered the largest asteroid. It is the only dwarf planet in the inner solar system.

Eris: A large Kuiper Belt object, Eris is roughly the same size as Pluto and was a key factor in the reclassification of Pluto as a dwarf planet.

Asteroids

Asteroids are rocky, metallic, or icy leftovers from the early Solar System, mostly found in the asteroid belt between Mars and Jupiter.

Composition:

- C-type: Carbon-rich, most common, located mainly in the outer belt, similar in composition to the Sun.

- S-type: Made of silicate rock and nickel-iron, found in the inner belt.

- M-type: Metallic, primarily iron and nickel.

- Other: Some are mixed materials or contain ice.

Location: Most orbit in the asteroid belt, 2-4 AU from the Sun.

Comets

Comets are small icy bodies from the outer Solar System, often called "dirty snowballs," made of ice, dust, and rock.

- Nucleus: Solid core of ice, dust, and rock, a few kilometers wide.

- Coma: Cloud of gas and dust that forms around the nucleus as it warms near the Sun.

- Tails:

-Ion Tail: Made of charged particles, points directly away from the Sun.

-Dust Tail: Made of small solid particles, curves due to solar radiation pressure.

- Composition: Water ice, frozen gases (carbon dioxide, methane, ammonia), dust, silicate minerals, and organic compounds.

Comets' origin

Comets come from two main regions of the Solar System: the Kuiper Belt and the Oort Cloud.

- The Kuiper Belt, located beyond Neptune (30-50 AU from the Sun), contains many icy bodies called Kuiper Belt Objects (KBOs), which are the source of most short-period comets.

- The Oort Cloud is a distant, spherical shell of icy objects that sends long-period comets into the inner Solar System, likely pushed there by early gravitational interactions with giant planets.

Meteoroid, Meteor, and Meteorite

These all refer to a small rock orbitting the sun. The rock is, however, at different stages of its journey: a meteoroid is in space, a meteor is the streak of light seen when it enters the atmosphere, and a meteorite is the fragment that survives the atmospheric entry and lands on the ground.

Meteoroid: A small rocky or metallic object traveling through space. Meteoroids can range in size from dust particles to the size of a small asteroid.

Meteor: When a meteoroid enters Earth's atmosphere at high speed, it burns up due to friction with the air, creating a bright streak of light we see as a meteor (or "shooting star").

Meteorite: If a meteoroid is large enough, it may not burn up completely during its atmospheric passage and a portion of it can survive to impact the Earth's surface. These surviving pieces are called meteorites

Extra-Solar Planets (Exoplanets)

Exoplanets are planets that orbit stars outside our Solar System. They vary widely in size, composition, and distance from their stars, providing insights into how planetary systems form and evolve. Scientists use several methods to detect and study these distant worlds.

Search Methods for Exoplanets: Doppler, Astrometric, and Transit/Eclipsing

Doppler Method (Radial Velocity):

- Detects tiny wobbles in a star's movement caused by the gravitational pull of an orbiting planet.

-These wobbles change the star's light spectrum slightly (redshift and blueshift).

-Best for finding large planets close to their stars.

Astrometric Method:

-Measures the star's precise position in the sky over time.

-A planet's gravity causes the star to move in a small orbit or "wobble."

-Works best for massive planets orbiting nearby stars but is technically challenging due to the tiny motion involved.

Transit/Eclipsing Method:

-Observes a planet passing in front of its star, causing a small, temporary dip in the star's brightness.

-Can reveal planet size, orbital period, and sometimes atmospheric composition.

-Works best when the planet's orbit aligns with our line of sight.

What is the current status of Exoplanet search?

-Thousands of exoplanets have been discovered, with over 5,000 confirmed planets as of 2025.

-Most were found using the Transit method (especially with NASA's Kepler and TESS missions) and Doppler method.

-Discoveries include gas giants, rocky Earth-like planets, and planets in habitable zones where liquid water could exist.

-New telescopes, like the James Webb Space Telescope (JWST), are now studying exoplanet atmospheres to search for signs of habitability or life.