Why are some Animals Born with their Eyes Closed? Jonathan

There are lots of examples of animals that are born with their eyes close - rabbits are just one example. It's all to do with whether animals need to be able to see when they are first born. Some animals, like horses or deer, are born with their eyes open because they have to be independent from birth. In the wild they have to be alert so that they can escape from predators or others dangers. But rabbits don't have to be able to see because they can rely on their parents looking after them until they are old enough to survive on their own. And since they are born in dark burrows anyway, there wouldn't be much point in them being able to see. Baby rabbits don't open their eyes for about a week after they're born. Kittens don't open their eyes for a week or so, and they can't see as well as a fully grown cat until they are several months old. Hares, though, are born with all their fur, and their eyes open !

March 2003

Why do my Dog's Eyes Shine When I Point a Light at Him? Claudia

Dogs', cats', horses' and cows' eyes glow when you shine a light on them because of a reflective layer of tissue at the back of the eye called the tapetum lucidum which is latin for Bright Carpet. The purpose of this layer is to help animals that come out at night, like dogs and cats, to see better at night. It works by acting as a mirror which reflects light hitting the back of the eye-ball back onto the light-sensitive part of the eye called the retina, giving the eye a second chance to pick up as much information from the light as possible. Some of the light reflected back in this way comes back out of the front of the eye causing the glowing effect that you see when you shine a bright light at your pet. During the daytime there's already too much light around to notice the effect. Humans don't have this reflective layer at the back of the eye and so when a bright light hits our eyes - like a camera flash - you often see people with 'red eyes' which is the camera flash lighting up the blood vessels at the back of the eye.

March 2003

How far down is it to the centre of the Earth ? Edward

The radius of the Earth, or in other words the distance to the centre, is 6,300 km or 3,963 miles. That means the Earth is 12,600km (7,926 miles in diameter) wide in total. The earth has 4 main parts, the crust, mantle and the inner and outer cores. The core is the biggest bit and is 3500 km or 2100 miles wide. It is between 4,300 and 7,200 degrees celsius in the middle of the earth which is hotter than the sun's surface. The inner core is solid iron and the outer core is liquid iron and sulphur. The mantle is between 100 to 250 km deep, and is 3700°C. It contains solid magnesium, iron, aluminium, silicon, and oxygen silicate compounds, which can move about in a plastic like way. The crust or top layer of the earth is the smallest part and is 6 to 11 km under the oceans, and 25 to 90 km thick under the land. This part contains calcium and sodium aluminium silicate minerals. The World's deepest hole is 40,000 feet deep (that's about 7.5 miles). It's in Russia, on the Kola Peninsula near the Norwegian border. They have been drilling since 1970, to gain a better understanding of the Earth's crust which is on average 20 miles thick. The previous deepest hole was in Oklahoma in the USA where a gas well stopped at 32,000 feet (6 miles deep).

March 2003