12.3 Earth in Space & the Universe

Day, Night and the Spinning Earth

Earth is a giant ball spinning in space. It rotates once on its axis - the imaginary line through the North and South Poles - every 24 hours. The side facing the Sun has daytime; the side facing away has night. As Earth spins, your town rotates into the sunlight (sunrise) and out of it (sunset). The Sun does not travel around us; it only looks that way because we are the ones moving.

Seasons: It's the Tilt, Not the Distance

The features of different seasons is a named topic on the Introductory paper, and it grows into one of the most-tested ideas in the senior papers. Earth also travels around the Sun - one orbit per year - and its axis is tilted at about 23.5 degrees. That tilt is the whole story of the seasons:

• When the Southern Hemisphere is tilted toward the Sun, sunlight strikes Australia more directly, days are longer, and we get summer - in December, January and February. At that same moment the Northern Hemisphere is tilted away and has winter.
• Six months later, in June, Earth has moved halfway around its orbit; now the Southern Hemisphere tilts away, sunlight arrives at a slant, days are short, and Australia has winter.

Here is the famous trap, and ICAS knows it: seasons are NOT caused by Earth being closer to the Sun. If distance were the cause, the whole planet would have summer at once - but Australia is hot while England is cold. In fact Earth is slightly closer to the Sun in January, during the Australian summer and the European winter, which proves distance is not the driver. Directness of sunlight and length of day, both caused by the tilt, are what matter.

Phases of the Moon

The Moon does not make its own light; we see it because it reflects sunlight. As the Moon orbits Earth about once a month, we see different amounts of its sunlit half: a thin crescent, a half moon, a full gibbous moon and a full moon, then shrinking back through a new moon when the dark side faces us. The Moon itself never changes shape - only our view of its lit half changes.

The Solar System

Our solar system is the Sun plus everything gravity holds around it. The eight planets in order from the Sun:

The inner four are small and rocky; the outer four are gas and ice giants. A memory sentence: My Very Excellent Mother Just Served Us Nachos. Between Mars and Jupiter lies the asteroid belt, and dwarf planets such as Pluto orbit further out.

Gravity, stars and galaxies

Gravity is the invisible pull between masses. It keeps your feet on the ground, keeps the Moon orbiting Earth, and keeps every planet orbiting the Sun instead of flying off into space. An orbit is really a continuous fall: a planet moves forward fast while gravity pulls it inward, and the two together bend its path into a loop.

The Sun is an ordinary star - one of hundreds of billions in our galaxy, the Milky Way - and there are hundreds of billions of galaxies in the observable universe. Stars look tiny only because they are unimaginably far away; even the nearest star beyond the Sun is so distant that its light takes over four years to reach us.

Investigating Other Worlds - Paper H

The problems of extraterrestrial investigation are a named topic on Paper H. Worked example: a Mars mission. Scientists want to know whether Mars ever had liquid water, so they send a rover. Every challenge shows up at once:

• Distance - Mars is tens of millions of kilometres away, so the trip takes months.
• Communication delay - radio signals take many minutes each way, so the rover cannot be joystick-driven in real time; it must make many decisions on its own.
• Harsh conditions - dust storms, freezing nights and radiation threaten every instrument.
• Cost - each mission takes years of planning and enormous funding, so a single failed landing loses everything.

Because of these problems, scientists test equipment endlessly on Earth, send robots before people, and treat every scrap of returned data as precious. When an ICAS question asks why a space investigation is designed a certain way, the answer almost always traces back to one of those four problems.

Senior extension: evidence for the formation of the Universe - Paper I

Senior papers connect space science to big-picture evidence. The leading scientific account is that the universe began expanding from an extremely hot, dense state about 13.8 billion years ago - the Big Bang theory. The evidence includes distant galaxies moving away from us (the universe is still expanding) and faint leftover heat radiation detectable in every direction of the sky. As with dating the Earth, the story is built from evidence and stays open to revision as better data arrives.