Seven lessons in a
free 3D model of space

Lesson 1 · How big is the Solar System?

In the app

• Earths between Earth and the Sun: about 11,700 (149.6 million km ÷ 12,756 km).

Ask the class

1. A peppercorn, about 2 mm across, some 24 metres away. A 22 cm football for a Sun 1,391,400 km wide makes Earth's 12,756 km a fifth of a centimetre, and its 149.6 million km 23.6 m.
2. To scale the planets would be specks too small to print, and a page wide enough for Neptune's orbit would leave Earth a dot near the middle. Books trade the true distances for pictures you can see.
3. About 4.2 years: Proxima Centauri is 4.2 light-years away, and a light-year is how far light goes in a year.

Worksheet

1. Mean distances from the Sun, as the planets' cards give them, in millions of km: Mercury 57.9, Venus 108.2, Earth 149.6, Mars 227.9, Jupiter 778.3, Saturn 1,426.7, Uranus 2,870.7, Neptune 4,498.4.
2. With Earth at 1 step: Mercury 0.4, Venus 0.7, Earth 1, Mars 1.5, Jupiter 5.2, Saturn 9.5, Uranus 19.2, Neptune 30.1. These steps are astronomical units.
3. Any reasoned answer. Mercury is small and lost near the Sun; Uranus and Neptune are tiny dots far apart.

Lesson 2 · Why the Moon changes shape

Ask the class

1. At full Moon the Sun is behind us: Earth sits between it and the Moon, so we face the Moon's lit side. At new Moon the Sun is behind the Moon, and we face its dark side.
2. Hardly ever. A crescent is close to the Sun in the sky, so it sets soon after sunset (the evening crescent) or rises shortly before sunrise (the morning one).
3. Earth shows phases too, the opposite of the Moon's: a full Earth at new Moon, a thin crescent of Earth at full Moon.

Worksheet

1. On the day you open this page the lab shows the Moon as it is tonight.
2. From the lab's calendar: the next full Moon is marked ● there, the next new Moon is marked ○.
3. We see half of the half the Sun lights, because the Moon is a quarter of the way round its orbit. Earth's shadow reaches the Moon only in a lunar eclipse, and only at full Moon.

Lesson 3 · Planets around other stars

Ask the class

1. The zone only says the starlight could keep water liquid on a surface. The planet also needs air of the right kind and enough mass to hold on to it. Venus lies near the inner edge of our Sun's zone and is 464 °C under thick carbon dioxide.
2. A transit dims the star by the share of its disc the planet covers, which gives the planet's size. The mass needs the star's wobble: a heavier planet tugs its star harder, and the star's light shifts as it moves toward us and away.
3. About 1,160 years from now: 579 years for our message to get there and as long for the reply.

Worksheet

The answers depend on the planet chosen. For Kepler-186 f the card gives:

1. Kepler-186 f, round the red dwarf Kepler-186, 579 light-years away.
2. Found in 2014 by the transit method, with the Kepler space telescope.
3. 1.17 times Earth's width; a year of 129.9 days.
4. For: it is about Earth's size and in its star's habitable zone. Against: it gets about a third of the light Earth gets, and its air is unknown. NASA's archive also flags the planet as disputed: its card says so.

Lesson 4 · Our galaxy and beyond

Ask the class

1. A light-year is how far light goes in a year, about 9.46 trillion km. It says how far away a thing is and, in the same number, how long its light has been travelling.
2. No: we see it as it was when the light left. A galaxy changes slowly, so it will look much the same, but anything that happened there since is still on its way to us.
3. We are inside the disc, so the Milky Way looks like a band across the sky, and dust hides most of the far side. Its shape comes from mapping the distances and motions of its stars and gas, and from comparing it with other galaxies. Orrery draws that map.

Worksheet

1. About 27,000 light-years: galaxy mode puts Sagittarius A* 26,996 light-years from the Sun.
2. For example: Neptune, about 4 hours; the Pleiades, 444 years (the light left around 1580); the Andromeda Galaxy, 2.6 million years in galaxy mode, where measurements give 2.4 to 2.6 million.
3. The note on arrival says: when earlier kinds of human were making stone tools. The first stone tools found so far are 3.3 million years old.

Lesson 5 · Landing at the Moon's south pole

Ask the class

1. At the pole the Sun never climbs more than a degree or two above the horizon, so the floors of deep craters are never lit. LRO measured some colder than −230 °C, cold enough to keep ice for billions of years. Near the equator the Sun stands high and the ground reaches about 127 °C by day.
2. Often Earth. From the pole Earth always sits low on the horizon, and for about half of each month it dips behind the hills, while the best ridges are sunlit most of the year. Each site's page in Landing sites gives both, month by month.
3. Orbiters found water ice in the polar shadows: LCROSS hit Cabeus crater in 2009 and saw it. Ice means water, air and rocket fuel made on the Moon, and the ridges beside it have sunlight for power. Landers can also now come down within a few hundred metres of a chosen point.

Worksheet

1. For example, Connecting Ridge CR1 at 89.45° S.
2. From the site's card for the chosen 30 days: 100% sunlit is 720 hours; a link open half the time is about 15 days. Its page lists the next twelve months.
3. Any advantage and danger that fit the numbers: long sunlight or a high ridge, against long blackouts from Earth, steep slopes or cold shadows close by.
4. Mark the reasons rather than the choice: a good answer uses the site's sunlight and Earth-contact figures.

Lesson 6 · Standing on Mars

Ask the class

1. About 30.8 Earth days: 30 × 24 h 40 min is 740 hours.
2. Weight is gravity's pull on your mass, and Mars pulls with 38% of Earth's force: it has a tenth of Earth's mass, and you would stand about half as far from its centre. You would feel a bit more than a third of your weight.
3. Phobos goes round Mars in 7.7 hours, faster than Mars turns (24.6 hours), so it overtakes the turning ground and crosses the sky from west to east. Our Moon takes a month, so Earth's turning carries it from east to west.

Worksheet

1. A day of 24.6 hours; a year of 687 Earth days.
2. About 38%, a bit more than a third; the 40 kg backpack would feel like about 15 kg.
3. Phobos: 7.7 hours (Deimos takes 30.3). Seen from the turning ground it crosses the sky about twice a Martian day.
4. A dusty, butterscotch sky by day with a small Sun; at night Phobos moving fast against the stars and Earth as a bright star near the Sun's side of the sky.

Lesson 7 · Kepler's laws with the star S2

In the app

• Halley's Comet is fastest at its closest to the Sun, 0.59 AU, and slowest at its farthest, 35.1 AU.

Ask the class

1. At its closest point to the black hole. As it falls in, gravity pulls it faster; as it climbs out, gravity slows it.
2. About 1,900 AU: at the two ends the speed times the distance is the same, so 16 times slower means 16 times farther. The orbit's measured far point is 1,950 AU.
3. Kepler's third law: the cube of the orbit's size divided by the square of its period gives the mass it goes round. For S2, 1,035 AU and 16.05 years give 4.3 million Suns.

Worksheet

1. 120 AU, at about 7,750 km/s as the race works it out from the orbit (the speed measured in 2018 was about 7,650 km/s).
2. 476 km/s, about 16 times slower.
3. A line from the black hole to the star sweeps out equal areas in equal times, so the star moves fast when close and slowly when far.
4. In 2034: May 2018 plus 16.05 years.