The SixthLens concept atlas
Follow your curiosity.
From a first question to the ideas beneath it. Explore 31 connected concepts through explanations, examples and interactive figures.
Find a concept ↓A path through the ideas.
All learning tracks →Plan a night under the stars
Connect sky brightness, target altitude and observing conditions before building a target list.
Follow this track →02 / 4 lessonsUnderstand the light you collect
Follow an image from telescope optics to photon noise and stacking.
Follow this track →03 / 5 lessonsUnderstand worlds in motion
Move from gravity and elliptical orbits to the changing viewpoint of a planet watcher.
Follow this track →Find your next question.
31 ideas, connected.
31 concepts in this view
Gravity
Every mass attracts every other mass with a force that falls off as the square of the distance between them.
A starting point / 6 follow-on conceptsExplore the concept →Kepler's laws
Orbits are ellipses with the Sun at one focus; a planet sweeps equal areas in equal times; and the square of the period is proportional to the cube of the semi-major axis.
Builds on 2 concepts / 4 follow-on conceptsInteractive figureExplore the concept →Eccentricity
How far an orbit departs from a circle, from 0 for a perfect circle to just under 1 for a very elongated ellipse.
A starting point / 1 follow-on conceptsExplore the concept →Escape velocity
The speed at which an object's kinetic energy exactly cancels the gravitational well it sits in, so it can leave and never return.
Builds on 1 concepts / 0 follow-on conceptsExplore the concept →Orbital resonance
Two orbiting bodies exerting regular, repeating gravitational tugs on one another because their periods form a ratio of small whole numbers.
Builds on 2 concepts / 0 follow-on conceptsExplore the concept →Tides
The stretching a body feels because gravity pulls harder on its near side than its far side.
Builds on 1 concepts / 2 follow-on conceptsExplore the concept →Precession of the equinoxes
The slow conical wobble of Earth's rotation axis, one turn every 25,772 years, driven by the Sun and Moon pulling on our equatorial bulge.
Builds on 1 concepts / 0 follow-on conceptsExplore the concept →Barycentre
The common centre of mass that two orbiting bodies both circle, rather than one circling the other.
Builds on 1 concepts / 0 follow-on conceptsExplore the concept →The inverse-square law of brightness
The apparent brightness of a source falls off as the square of its distance, because the same light is spread over an expanding sphere.
A starting point / 5 follow-on conceptsExplore the concept →Blackbody radiation
The characteristic spectrum any warm, opaque object emits, whose shape depends only on its temperature.
A starting point / 4 follow-on conceptsExplore the concept →Wien's displacement law
The wavelength at which a blackbody radiates most strongly is inversely proportional to its temperature.
Builds on 1 concepts / 1 follow-on conceptsExplore the concept →Spectral lines
Sharp bright or dark features in a spectrum, produced when electrons in atoms jump between fixed energy levels.
Builds on 1 concepts / 2 follow-on conceptsExplore the concept →Doppler shift and redshift
Motion along the line of sight stretches or compresses the wavelength of light, moving spectral lines toward the red or the blue.
Builds on 1 concepts / 1 follow-on conceptsExplore the concept →Extinction and airmass
The dimming and reddening of starlight by the atmosphere, which increases with how much air it has to cross.
Builds on 1 concepts / 1 follow-on conceptsExplore the concept →The magnitude scale
A logarithmic, backwards brightness scale in which smaller numbers are brighter and five magnitudes is a factor of exactly 100.
Builds on 1 concepts / 1 follow-on conceptsInteractive figureExplore the concept →Spectral classification
The sequence O B A F G K M, ordering stars from hottest and bluest to coolest and reddest by the lines in their spectra.
Builds on 2 concepts / 1 follow-on conceptsExplore the concept →The Hertzsprung–Russell diagram
A plot of luminosity against temperature on which stars fall into a few narrow bands rather than scattering at random.
Builds on 3 concepts / 0 follow-on conceptsExplore the concept →Stellar parallax
The tiny annual shift in a nearby star's apparent position, caused by the Earth viewing it from opposite sides of its orbit.
Builds on 1 concepts / 1 follow-on conceptsInteractive figureExplore the concept →The cosmic distance ladder
The chain of overlapping techniques by which distances are measured, each calibrated against the shorter rung below it.
Builds on 3 concepts / 0 follow-on conceptsExplore the concept →Albedo
The fraction of incident sunlight a surface reflects rather than absorbs.
A starting point / 1 follow-on conceptsExplore the concept →Synodic period
The time between successive identical alignments of two bodies as seen from a third — for a planet, the gap between one opposition and the next.
Builds on 1 concepts / 1 follow-on conceptsExplore the concept →Retrograde motion
The apparent backwards loop a superior planet traces against the stars as the Earth overtakes it on the inside.
Builds on 1 concepts / 0 follow-on conceptsExplore the concept →Differentiation
The separation of a molten young world into layers by density — iron sinking to a core, lighter silicates floating to a crust.
Builds on 1 concepts / 0 follow-on conceptsExplore the concept →Equilibrium temperature
The temperature a world settles at when the sunlight it absorbs exactly balances the heat it radiates away.
Builds on 3 concepts / 0 follow-on conceptsExplore the concept →Aperture and resolving power
The finest detail a telescope can separate, set by diffraction and therefore by the diameter of its objective alone.
A starting point / 2 follow-on conceptsExplore the concept →Focal ratio and image scale
Focal ratio is focal length divided by aperture; image scale is how many arcseconds of sky land on one pixel.
Builds on 1 concepts / 0 follow-on conceptsExplore the concept →Seeing
The blurring of a point source by atmospheric turbulence, measured as the full width at half maximum of the resulting star image.
Builds on 2 concepts / 0 follow-on conceptsExplore the concept →Signal, noise and stacking
Combining many exposures so that the signal adds linearly while random noise adds as the square root, improving the ratio as √N.
Builds on 1 concepts / 0 follow-on conceptsInteractive figureExplore the concept →Photon noise
The irreducible grain in any measurement of light, arising because photons arrive at random and counting N of them carries an uncertainty of √N.
Builds on 1 concepts / 2 follow-on conceptsExplore the concept →Light pollution and the Bortle scale
A nine-point ranking of night-sky brightness, from a pristine site where the Milky Way casts shadows to an inner city where a handful of stars survive.
Builds on 1 concepts / 0 follow-on conceptsExplore the concept →The discovery of Neptune
A planet found in 1846 by mathematics rather than by searching — predicted from discrepancies in Uranus's orbit, then located within one degree on the first night of looking.
Builds on 2 concepts / 0 follow-on conceptsExplore the concept →
See how the ideas connect.
The map traces prerequisites. The library above provides the same concepts as keyboard-accessible links.