19 topics
Astronomy help, from moon phases to the distance ladder
Astronomy asks you to hold three positions in mind at once — where the observer is standing, where the object is, and which way the sunlight is arriving — and phases are where that first collapses. Half the Moon is lit at all times. What changes through the month is how much of the lit half is turned toward us, and every phase question is really that one geometry question in disguise.
Where students get stuck
If the Moon orbits monthly, why isn’t there an eclipse every month?
Because the Moon’s orbit is tilted about five degrees to the plane of Earth’s orbit. At most new moons the shadow passes above or below us entirely, so nothing happens. The two points where the tilted orbit crosses that plane are called nodes, and an eclipse can only occur when a new or full moon happens to land near a node. Those alignments come round roughly twice a year, which is why eclipses arrive in seasons rather than monthly. Solar eclipses need a new moon at a node, lunar eclipses a full moon at a node.
Kepler’s laws are words to me — what do I actually do with them?
Each law is an instruction. The first says orbits are ellipses with the Sun at one focus, so a planet has a near point and a far point rather than a fixed radius. The second says a line to the Sun sweeps equal areas in equal times, which in practice means speed is fastest at perihelion and slowest at aphelion, and you use it to compare speeds at two points on the same orbit. The third gives you numbers: period squared equals semi-major axis cubed, provided the period is in years and the distance in astronomical units. Mixing units is the most common lost mark on the entire topic.
The H-R diagram looks backwards and I can’t read it
It is backwards, deliberately. Temperature increases to the left, so hot blue stars sit on the left and cool red ones on the right, and luminosity climbs upward. The diagonal band running from upper left to lower right is the main sequence, and it is really a mass sequence: heavier stars are hotter and brighter and burn through their fuel faster. The clump at upper right is giants — cool but enormous, so bright despite the low temperature. The scatter at lower left is white dwarfs, hot but tiny. A star does not slide along the main sequence as it ages; it sits on it, then leaves it.
Parallax, light-years, parsecs — which one do I use when?
Distance in astronomy is a ladder, and each rung has a range. Parallax measures the tiny shift of a nearby star against the background as Earth moves across its orbit, and it works out to a few thousand light-years before the angle gets too small to trust. A parsec is simply the distance at which that shift is one arcsecond, about 3.26 light-years, so distance in parsecs is one divided by the parallax angle in arcseconds. Beyond parallax you switch to standard candles, objects with a known true brightness such as Cepheid variables, then to supernovae, then to redshift. Each rung is calibrated by the one below it.
What's covered
Astronomy topics you can work through with a tutor, generate practice on, or turn into flashcards and a study plan.
The sky from Earth
- The celestial sphere, altitude and azimuth
- Daily and annual motion of the stars
- Seasons, axial tilt and solar altitude
- Moon phases, lunar eclipses and solar eclipses
- Retrograde motion and the shift to a heliocentric model
Gravity and orbits
- Kepler’s three laws
- Newtonian gravitation and orbital speed
- Tides and tidal locking
- Escape velocity and transfer orbits
The solar system
- Formation from the solar nebula
- Terrestrial and jovian planets compared
- Asteroids, comets and meteor showers
- Moons, ring systems and dwarf planets
Stars, galaxies and cosmology
- Light, the electromagnetic spectrum and spectral classes
- Apparent and absolute magnitude, parallax and the distance ladder
- The Hertzsprung-Russell diagram
- Stellar life cycles, supernovae, neutron stars and black holes
- Galaxy types, redshift and Hubble’s law
- The Big Bang and the cosmic microwave background
Astronomy questions
My star chart assignment is due and it has been cloudy all week. Can it help?
It can work with the chart itself. Share your screen or hold the printed chart up to the camera, and it will walk you through what should be above the horizon at your latitude, date and time, and why a given constellation has already set. That gets the reasoning done, though for an observation log you still need a clear night and your own recorded times.
Do I need calculus to handle the orbital questions?
Not for most first courses. Kepler’s third law, orbital speed and gravitational force are algebra with careful unit handling, and unit conversion is where the marks actually go missing. If your course does use calculus for orbital energy or escape velocity, the tutor works those derivations on the whiteboard line by line.
Can I just read out the quiz questions and have it answer them?
It will not run through a graded quiz as an answer key. Ask it a phases or eclipse question and it draws the Sun, Earth and Moon on the whiteboard and asks you which side is lit before it says anything. If you want to check work you have already finished, paste your answers and it will tell you which step went wrong rather than replacing it.
Stuck on astronomy right now?
Talk it through out loud, share your screen, and watch it worked out step by step on a whiteboard.
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