Astro Mega File (PDF)

*What did I learn today about the Earth as a planet?* 1. The Earth is a **rocky pla**net, **one astronomical unit** from the Sun. 2. The **diameter of the Earth is 13 000km**, and it is made up from five main layers. 3. The outer layer is a thin atmosphere/ocean. The gas part extends...

*What did I learn today about the Earth as a planet?* 1. The Earth is a **rocky pla**net, **one astronomical unit** from the Sun. 2. The **diameter of the Earth is 13 000km**, and it is made up from five main layers. 3. The outer layer is a thin atmosphere/ocean. The gas part extends about 100km upwards, but it gets exponentially thinner as you gain altitude, so the part that humans can breathe is really only the bottom 3-5km. The ocean gets to a maximum depth of about 8km. 4. The rocky layers are liquid or solid depending on three interacting factors; a. **Pressure** -- pressure increases with depth, high pressure raises melting points, so layers deep underground may be solid, even though they are extremely hot. b. **Temperature** -- temperature increases with depth. High temperature melts layers if it is higher than the pressure-related melting point. c. **Elements present** -- silicate rock melts at a lower temperature and has lower density than iron-rich material. In the young molten Earth, nearly all of the iron sank into the centre, giving us an iron core and a silicate exterior. 5. The first "rocky" layer is the **crust**. This is thick on land (about 80km) and thinner under the oceans (about 8km). It is composed mostly of silicate rock, rich in lighter elements like calcium, magnesium aluminium etc. 6. Under this layer is the **mantle** -- about 2800km thick. It is solid near the bottom, but can flow slowly (we call this plastic), and becomes more like a liquid as it rises to the lower pressure regions under the crust. 7. The bottom of the crust and the top of the mantle are solid, and linked together. They are separated by a layer/boundary called the moho, which is marked by a change in chemical composition. 8. Beneath this, the mantle is more plastic than solid -- partially melted. 9. Heat rises up in the plastic mantle via convection plumes from deeper in the Earth. Heat plumes reaching the crust may cause volcanoes, and push tectonic (crust) plates around. 10. The next layer is the **outer core**. This is mostly iron, and the temperature is high enough to melt it into a liquid. 11. The final layer is the **inner core**. This is pure iron, and the pressure is high enough to stop it melting, despite being at about 5000 deg C. What did I learn today about sidereal and synodic days and moon orbits? 1. The stars are an extremely **long way** away from the Earth. 2. This means that on the scale of the Earth-Moon-Sun system, the **angle to them never really changes** that much. 3. Thus, if you measure the orbit of the Moon by comparing it against the background of the stars, you'll get a value of **27.3 days**. 4. This is called the Moon's **sidereal period** 5. Sidereal means "**relative to the fixed stars**". 6. If you measure the Moon's orbit by comparing it to the relative positions of the Earth and the Sun (i.e. by looking at time between the **same two phases**), you'll get an orbit that lasts 29.5 days. 7. This is because the Moon has to spend **two days** "catching up" on the Earth's orbital motion. 8. This is called the Moon's **synodic period**. It's the one we all use in our calendars. 9. The Earth orbits the Sun once every **365.25** days. (sidereal) 10. That\'s roughly **one degree of orbit per day**, because a circle has 360 degrees. 11. We can **pretend** that the Sun orbits the Earth - it feels the same. 12. This means that the **Sun travels 1 degree across the sky** background each day. It moves in an eastwards direction against the fixed stars. 13. In a single day though, the Sun seems to move westwards, because of the **Earth's daily rotation**. It rises in the east and sets in the west. 14. At **local noon**, the Sun is **due south**, and at its **highest point** in the sky. 15. We actually call this **culmination**. The Sun culminates at **local noon**. 16. On each rotation of the Earth, the Earth has to rotate through **one degree extra** to make up for the fact that the Sun has moved. 17. This takes about **four minutes**. 18. So, a **sidereal** day for the Earth (relative to the distant stars) takes 23 hours and 56 minutes, but a **synodic** day (culmination to culmination) takes an extra four minutes = 24 hours exactly. *What did I learn today about the Moon's surface?* 1. The moon formed from **ejected material** after the Earth and a Mars-sized body (**Theia**) collided about 5.3 byr ago. 2. Evidence for this collision is seen in the **dehydrated** nature of the lunar soil (regolith), the imbalance of "light" **zinc** isotope compared with "heavy" zinc, the abundance **of kreep-rich** rocks. 3. The most obvious thing you can see about nearside is that there are **flat-looking dark areas** **called seas** (sing. Mare, pl. maria) and **pale rough areas called highlands**. 4. The highlands are mostly made out of **overlapping craters**. 5. **Craters** are very common round features, made when a **rocky object** hits the Moon at very **high speed** (10 -- 30km per second is typical). 6. The kinetic energy of the impact determines the size and depth of a crater, but they are almost all circular because the shockwave that makes them travels equally in all directions from the point of impact. 7. Craters have rim walls, thrown up above the surrounding plain by the impact. 8. Craters can be classified by size a. **Simple craters** are small (\