Charting the Heavens Pt.1
Why Study Astronomy
Astronomy is the most fundamental and the oldest science.
Astronomy promotes constructive thought.
This is the space age!
Astronomy and Distance
Astronomy is the study of the universe.
Distances are extremely large and are commonly expressed in light-years and astronomical units (AUs).
Light-year: the distance light travels in one year.
1\,\mathrm{ly} \approx 9.46\times 10^{12} \ \mathrm{km}
(approximately 5.88\times 10^{12}\ \mathrm{mi})
AU: the average distance between the Earth and the Sun.
1\,\mathrm{AU} \approx 1.496\times 10^{8} \ \mathrm{km}
Our Place in Space
Earth is an ordinary rocky planet that orbits the Sun in our solar system.
You are here: Earth’s location within the solar system.
Our Solar System
Definition: a collection of objects orbiting the Sun, our star.
Age: about 4.5\times 10^{9}\ \mathrm{years} (4.5 billion years).
The Sun is big; the Earth is small in comparison (Earth compared to the Sun).
Contains: 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).
Also includes numerous dwarf planets, moons, asteroids, and comets.
Our Galaxy: Milky Way
Galaxy: a huge collection of billions of stars.
Milky Way: our galaxy, one galaxy among billions of others in the observable universe.
The Universe
Universe: the totality of all space, time, matter, and energy.
A Few Important Distinctions
The Earth is a planet in our solar system.
Our solar system is just one of billions in our galaxy, the Milky Way.
The Milky Way is one galaxy among billions of galaxies in the universe.
The universe encompasses all time, space, matter, and energy.
Scientific Theories
In science, a "theory" is a well-established framework, not a guess.
Examples: theory of gravity, etc.
Characteristics of scientific theories:
Must be testable.
Backed by multiple lines of evidence.
Studied by many people over a long period of time.
They can be proven wrong, but can never be proven right with 100% certainty.
The Night Sky
The simplest astronomical observation: look up.
Roughly 3000 stars are visible at any one time.
Stars appear distributed randomly, but humans tend to see patterns.
Examples: Orion.
Constellations
Stars are grouped into constellations: figures with meaning to those who group them (e.g., Orion).
Why Some Stars Appear Close in the Sky But Not in Space
The sky shows a projection of space; stars that look near each other can be far apart in actual 3D space.
The seven prominent stars form patterns (e.g., asterisms such as the recognizable shapes we call constellations).
The Celestial Sphere
The celestial sphere is a representation of how the sky looks from Earth.
Stars appear to lie on the inner surface of an imaginary sphere surrounding the Earth.
In reality, stars are at vastly different distances; we use a 2D spherical coordinate system to locate objects.
Celestial Coordinates
Celestial latitude = declination (δ).
North of the celestial equator is positive (+).
South is negative (−).
Celestial longitude = right ascension (α).
Earth’s Orbital Motion and the Night Sky Through the Year
The Sun moves through twelve constellations over the year; these form the zodiac.
From the dark side of Earth, our view of the night sky changes as the planet moves in its orbit around the Sun.
Example constellations mentioned (across seasons): Deneb, Vega, Castor, Regulus, Capella, Altair, Betelgeuse, Rigel, Sirius, Antares, etc., placing different stars in the sky at different times of year.
Diagrams show seasonal views of the southern horizon in summer and winter.
What Causes Earth’s Seasons?
Earth’s axis is tilted by 23.5^{\circ}.
The axis always points in roughly the same direction as we orbit the Sun, which causes the seasonal differences.
What Causes Earth’s Seasons? (Expanded)
The tilt of Earth's axis causes areas of the earth to receive different amounts of sunlight during the year.
When tilted toward the Sun, sunlight is more direct.
This leads to summer.
When tilted away from the Sun, sunlight is less direct.
This leads to winter.
Seasons are caused by axial tilt, not by changes in distance to the Sun.
Seasons: Key Points
Solstices and equinoxes mark seasonal boundaries:
Summer Solstice: June 21
Vernal (Spring) Equinox: March 21
Autumnal (Fall) Equinox: September 21
Winter Solstice: December 21
Seasonal patterns show smaller and larger ground areas covered by sunlight over the year due to tilt.
Illustration labels: (Summer solstice) smaller ground area impact; (Winter solstice) larger ground area impact; the Sun’s altitude changes with the season.
Precession
Precession is the slow rotation of Earth’s axis itself, like a spinning top.
It completes one full cycle in about 2.6\times 10^{4}\ \text{years} (≈ 26,000 years).
Current pole star is Polaris; future pole star will be Vega (in A.D. 14,000); past pole star was Thuban (around 3000 B.C.).
The axis traces a circle relative to the celestial sphere, changing which star aligns with the pole over millennia.
Polaris, Vega, Thuban; Pole Star History
Polaris: current North Pole star.
Vega: pole star in the year AD 14,000.
Thuban: pole star around 3000 BC.
The Earth’s axis tilt is 23.5° (as shown in the diagrams).
Ecliptic plane and equator are shown to illustrate the axis and rotation geometry.
Our Solar System: Composition and Scale (Slide prompts)
Question: OUR SOLAR SYSTEM CONSISTS OF _.
Options include:
The Sun and the planets, and nothing else.
The Sun and all the objects that orbit it.
A few hundred billion stars, bound together by gravity.
The Sun and several nearby stars, as well as the planets and other objects that orbit these stars.
Correct answer: The Sun and all the objects that orbit it.
Order of Size: Smallest to Largest
The correct order from smallest to largest is:
Planet < Solar System < Galaxy < Universe.
This helps situate objects and systems within the cosmos.
Key Takeaways
The universe is hierarchical: planets, solar systems, galaxies, universes.
The night sky changes with time due to Earth’s rotation and its orbit around the Sun.
Constellations and the zodiac help us organize sky observations, while celestial coordinates allow precise locations.
Seasonal changes arise from axial tilt, not distance to the Sun, with notable dates marking solstices and equinoxes.
Precession gradually changes the pole star over millennia, altering the identity of the North Star across eras.
Scientific theories are robust, testable, and built on extensive evidence, yet remain open to revision with new data.