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Estimating the universe's ordinary matter without counting atoms

Nobody could ever count the atoms in the universe one by one, yet physicists often quote an estimate of roughly 10⁸⁰ atoms' worth of ordinary matter in the observable cosmos. Rather than counting individual particles, cosmologists work top-down: they measure the size and expansion of the observable universe, calculate the fraction made of ordinary matter rather than dark matter, and divide by the mass of a proton.

3–7 min

Questions you could explore

  • What observations help estimate the density of ordinary matter?
  • How would the answer change if you counted nuclei rather than intact atoms?

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Space

A crescent Moon is not a partly switched-off Moon

Sunlight illuminates half the Moon, while its changing position around Earth changes how much of that lit half we can see. A phase is different from an eclipse, where one body blocks light from reaching another.

3–5 min

A crescent Moon is not a partly switched-off Moon

Space3–5 minSunlight illuminates half the Moon, while its changing position around Earth changes how much of that lit half we can see. A phase is different from an eclipse, where one body blocks light from reaching another.

Space

Two eclipses, two different shadows

A solar eclipse and a lunar eclipse both involve the Sun, Earth, and Moon, but the shadow falls in a different place. In one, the Moon’s shadow reaches Earth; in the other, the Moon moves into Earth’s shadow. The Moon’s tilted orbit helps explain why these alignments do not happen every month.

5–7 min

Two eclipses, two different shadows

Space5–7 minA solar eclipse and a lunar eclipse both involve the Sun, Earth, and Moon, but the shadow falls in a different place. In one, the Moon’s shadow reaches Earth; in the other, the Moon moves into Earth’s shadow. The Moon’s tilted orbit helps explain why these alignments do not happen every month.

Space

What can century-old photographs reveal about stars today?

A star photographed on a glass plate a century ago can become a new measurement when compared with later observations. Harvard's DASCH project digitizes an astronomical plate archive, alongside dates and logbooks. Explore how calibration makes those old images useful for investigating changes across decades.

8–10 min

What can century-old photographs reveal about stars today?

Space8–10 minA star photographed on a glass plate a century ago can become a new measurement when compared with later observations. Harvard's DASCH project digitizes an astronomical plate archive, alongside dates and logbooks. Explore how calibration makes those old images useful for investigating changes across decades.

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