Sun, Moon, and Stars: Astronomy for Beginners · Part II, Ch. VI, part 1 · Part 2: Part II: A closer look at the Solar System
18. Mercury and Venus up close
Goal: Your child can describe Mercury and Venus as we know them now, explain what a transit is, and say why Venus is the brightest planet in our sky.
You'll need
- The book, open to Part II, Chapter VI
- A lamp, a ball and the darkened room from the Moon-phase lesson
- The planet chart
What's in the book
The chapter revisits Mercury (fast, with a very oval orbit and a blazing Sun) and Venus (Earth's 'twin', with phases like the Moon). It explains transits, when Mercury or Venus crosses the face of the Sun as a black dot, and how the transits of Venus helped measure the Sun's distance. It ends with guesses about the length of their days and their climates.
The lesson, step by step
Read together
Read from the start of the chapter to the part about the climate of Venus. Stop at the list of transit years: 1761, 1769, 1874, 1882.
Venus has phases
Use the lamp and ball again, but this time the ball goes round the lamp, inside your own circle, while you watch from outside. You see a thin crescent when it is nearly between you and the lamp, a half when it is off to the side, and a small full disc when it is far beyond the lamp.
SayThat is why Venus looks brightest when it is a crescent or half: it is much nearer to us then.
A black dot on the Sun
Safety first: never look at the Sun, not even for a moment, and not through sunglasses, smoked or darkened glass, film, binoculars or a telescope. It can hurt your eyes for good, and it does not hurt while it happens.
SayIn a transit, Venus or Mercury looks like a tiny black dot crossing the Sun. Astronomers watched it with special telescopes and filters, or by projecting the Sun's image onto a white screen. We look at photographs.
What we know now
Mercury does not keep one face to the Sun, as Schiaparelli thought. It turns three times for every two trips round the Sun. With no air to hold heat, its ground is hotter than an oven by day and far colder than any place on Earth by night.
Venus turns very slowly, and backward; its day is longer than its year. It is hidden under thick clouds of acid, and its air is so thick and full of carbon dioxide that it traps heat like a blanket: Venus is the hottest planet, hot enough to melt lead. The book was right that an atmosphere keeps a planet warmer.
Radar has mapped Venus' mountains through the clouds; the highest is taller than Mount Everest, though not three or four times taller.
The last transits of Venus were in 2004 and 2012. The next pair will not come until 2117 and 2125.
Tell it back
Ask your child to describe, as a space travel guide would, what a visitor to Mercury or to Venus would find.
Check questions
What is a transit?
Why are transits of Venus so rare?
Why do Mercury and Venus show phases like the Moon?
Notice it this week
If Venus is an evening star now, look for it after sunset each clear evening and note whether it is getting higher or lower in the sky from week to week. Look only after the Sun has fully set.
Afterwards
Update the planet chart with one new fact each for Mercury and Venus.
Answers to the book's problems
A transit is when Mercury or Venus passes exactly between us and the Sun and is seen as a small black dot crossing the Sun's face. Because their orbits are tilted a little from ours, so they usually pass just above or below the Sun. Because they go round the Sun inside our orbit, so we see different amounts of their sunlit side.
Worked out for this site from the scan. If a number in your copy differs, trust the book.