Cosmic Distance Scales - ANSWER KEY – Page 1 –

  1. The Earth’s radius of about 6370 kilometers was calculated correctly by Eratosthenes and later confirmed by satellites.

True: With this information, Eratosthenes inferred that the Earth's radius was 6366 km. Both of these values are very close to the accepted modern values for the Earth's circumference and radius, 40,070 km and 6378 km respectively, which have since been measured by orbiting spacecraft.

  1. Distances between objects in the solar system are commonly measured using gigameters.

False: Distances in the solar system are commonly measured in Astronomical Units (AU). An AU is simply the average distance between the Earth and the Sun.

  1. Johannes Keplerfigured out that all planets in the solar system moved the same speed and that Mars was exactly 2,331,670 kilometers from Earth.

False:

The closer the planet was to the Sun, the faster it moved.

Though he wasn't able to come up with distance measurements in kilometers, Kepler was able to order the planets by distance and to figure out their proportional distances.

  1. Astronomers can use the apparent shifting location of an object to determine the distance to our nearest star, and many others.

True: Astronomers can measure parallax by measuring the position of a nearby star very carefully with respect to more distant stars behind it, then measuring those positions again six months later when the Earth is on the opposite side of its orbit.

  1. Stars are constantly moving and will not remain in the same location as time passes.

True: -The distances to the stars in our solar neighborhood will change over time, because the stars are not stationary, as we discussed on the Nearest Stars page.

-This motion of the stars can alter the way constellations look from Earth.

  1. The inverse-square law tells us that the brightness of a star will decrease as distance increases, so we can use this to calculate distances to stars and other galaxies.

True: The inverse-square law says that the flux from a luminous object decreases as the square of its distance.

-If we know the luminosity of a star (for instance, we have a measured parallax for one star of the same type and know that others of the same type will have similar luminosities), we can measure its apparent brightness and then solve for its distance.

  1. Most of the closest galaxies are approximately 1000 light years away.

False: The closest known galaxy to us is the Canis Major Dwarf Galaxy, at 236,000,000,000,000,000 km (25,000 light years) from the Sun. The Sagittarius Dwarf Elliptical Galaxy is the next closest , at 662,000,000,000,000,000 km (70,000 light years) from the Sun.

-The Large Magellanic Cloud, 1,690,000,000,000,000,000 km (179,000 light years) away, was once considered the nearest galaxy outside of the Milky Way. The Small Magellanic Cloud is 1,980,000,000,000,000,000 km (210,000 light years) away.

Cosmic Distance Scales – Page 2 – ANSWER KEY

  1. Studying stars and galaxies in the local group is hard because it is hard to make out individual objects at such massive distances.

True: It's not just the apparent faintness of stars at such large distances; the crowding of stars in a small patch of the sky is a more important problem.

-So each pixel covers 2.5 by 2.5 arcseconds (times the 'thickness' of the galaxy) - which is likely to contain several stars at least. This makes it more difficult to study individual objects!

  1. Nearest Superclusters: Light waves will appear to shift towards the blue wavelength when they are moving away from us, and we know they appear more blue the farther away they are, thanks to a law proposed by Edwin Hubble.

False: If a galaxy is moving away from us, the "colors" of the absorption lines that we see here on Earth get shifted toward the longer, red wavelengths of light. This is what we call redshift. If a galaxy is moving towards us, its spectrum becomes bluer, or blueshifted.

  1. The Farthest Visible Reaches of Space:The Hubble Ultra Deep Field took pictures of objects whose light left them over 13 billion years ago.

True: The extreme distance of these newly discovered galaxies means their light has been traveling to us for more than 13 billion years, from a time when the Universe was less than 4% of its current age.