Earth and the Solar System

A light-year is a unit of distance, not time: "the time for light to cross the galaxy" scores zero. The two elements are (1) distance and (2) travelled by light in one year. Value: $1 \text{ ly} \approx 9.46 \times 10^{15}$ m — and the unit is metres, not km (a common MCQ trap: right number, wrong unit).

A planet's surface gravitational field strength $g$ depends on the planet's own mass (and radius), NOT its distance from the Sun. "Mars has weaker gravity because it is further from the Sun" is wrong. Earth has more mass than Venus, so Earth's surface $g$ ($\approx 10$ N/kg) exceeds Venus's ($\approx 8.9$ N/kg). Definition: $g = F/m$, unit N/kg.

When asked what redshift is, "the Doppler effect" is the common wrong answer and earns no credit. Redshift is the increase in the observed wavelength of light from a galaxy moving away from us (spectral lines shifted toward the red end). The Doppler effect is the underlying mechanism, but naming it alone does not define redshift.

Keep the hierarchy straight: the Universe is billions of galaxies; a galaxy is billions of stars; the Solar System is one star (the Sun) with its planets. Writing "the Universe is billions of stars" describes a galaxy. Many candidates cannot distinguish the Universe from a galaxy — name the correct level for every mark.

In $v = 2\pi r / T$ (and light-travel sums), the period $T$ must be in seconds. Convert days to seconds first: multiply by $24 \times 3600 = 86400$. Leaving $T$ in days (e.g. using 365 days directly) gives an answer out by a factor of 86 400. Always convert time to SI units before substituting, and quote the unit.

CMBR formed shortly after the Big Bang (when the Universe first became transparent), not "at the Big Bang". An answer that just says "Big Bang" with no reference to time is not creditworthy. As the Universe expanded, this radiation was redshifted from high energy to the microwave region, and it now arrives from all directions.