Similarly, when you look out at Great Barrier Island from Sacred Heart on a clear day the two separate highlands on the island (80 km away) appear to be separate islands. If you do the maths to calculate what you should see based on the radius of the Earth, it exactly corresponds to what you actually see:

The view above (photo by Br Anthony from P10) shows Aotea/Great Barrier Island as viewed from the top floor Pompallier Block at Sacred Heart College. The distance to Mt Hobson is 80 km. If you calculate what a person should see from this height at this distance, anything below the 280 metre contour line should be hidden. There is a pass between these two mountains, although you have to climb to 120 m to go from Whangaparapara on the Auckland side of the island to Claris on the far side. As predicted, this is below the horizon.

This video is a "debate" between a science presenter and a 'Flat Earther'. It is interesting in particular for the illustrations about the way that people overestimate their own level of knowledge about a subject.

The shape of the Earth also allows us to explain why shadows get longer as you move away from the equator:

This can also be tested by observations - over a longer distance, though. For example, by coincidence St Bede's (Marist Brothers college in Lower Hutt) lies 490 km exactly south of Sacred Heart. Some students measured the angle of the Sun at exactly the same time on equinox day (this day was chosen for a reason you'll find out late). The Sacred Heart boys got 53.5° . The St Bedes boys got 48,9° , a difference of 4.6 degrees.

Lets test this: if the Earth is round, then this means SHC and SBC are 4.6° apart on the line of longitude passing around the Earth and through both schools.

That would mean the circumference of the Earth is

490 km x (360° ÷ 4.6°) = 38,265 km.

This is fairly close to the actual figure of a little over 40,000 km that is the accepted by the round earth 'conspiricy'. I wonder why?

Going around the Sun

Another old idea is one that stayed around form much longer: that the Sun goes around the Earth. This is also wrong. The Earth revolves around the Sun while it rotates on its axis.

We call the place where the Earth's axis touches the surface the poles and the imaginary circles around the poles are lines of latitude. The line of latitude halfway between the poles is called the equator. Imaginary lines from pole to pole are called lines of longitude. The direction of sunrise is east and sunset is west.

This means that you are travelling from west to east when you are standing on Earth's surface. Your speed depends on your latitude - the closer you are to the equator the faster you go. A person standing on the the axis at one of the poles is just turning around once every 24 hours.

The side of the Earth facing the Sun is lit up, and experiences day; the other side is night. The image below is captured from the Himawar 8, a Japanese weather satellite that is in geostationary orbit above New Guinea. The image was captured at 10 am (NZDT) on the 3rd of April.

The partly lit line between day and night is called the terminator. While you are travelling through the terminator as Earth rotates, you experience partial sunlight called twilight (at dawn and dusk). People at the equator are going much faster, so for them the twilight time is very short. By contrast, people at high latitudes (nearer the poles) can have a twilight that lasts for an hour or more.

You can see on the image above that at 10 am NZDT at this time of year Auckland is fully in daylight. Sydney,will just be starting to see the first bits of sky getting light in the east. Perth will still be fully in the dark. This is why we have different time zones at different longitudes.

Seasons

Below are four Himawari images, each captured mid-morning NZ time at four different dates:

You will see in the December image that Antarctica is lit up (and would stay lit 24 hours a day on that date) and the North Pole is in darkness. The June image is the opposite - the North Pole is lit up and Antarctica is in (permanent) darkness on that day.

The March and September images are more-or-less identical. On these dates, the terminator runs exactly through the North and South poles. These dates are called the equinoxes. The June and December dates are times of maximum light or darkness at the poles; they are called solstices.

The reason this happens is because the Earth's axis is tilted about 23° from the ecliptic. Instead of always pointing towards the Sun, the tilted axis keeps pointing the same way as the Earth goes around the Sun:

During the December solstice, NZ travels much further through the daylight part of the Earth than through the dark of night, The point opposite NZ in the Northern Hemisphere (in Spain) does the opposite. This means the day is longer than the night in NZ; we have more time to heat up than cool down so the average temperature over 24 hours is higher than the yearly average. In Spain, it will be the opposite: night is longer than day, they have longer to cool down and the daily average temperature will be cooler than the yearly average.

The opposite applies in June: NZ days are short, nights are long. In Spain, the days are long and the nights short.

(Note: points opposite each other on the globe are called antipodes. The antipodal point for Sacred Heart is near a village in Spain called Setenil de la Bodegas. It is quite an interesting place with some of the houses built into natural caves - have a look at the link and in Google Earth.)

In the parts of the world which are away from the equator this produces very noticeable changes: day lengths change and temperatures rise and fall. We call these changes seasons. The diagram below shows the seasons for NZ related to our position around the Sun:

Be aware that many diagrams you see on the internet are based on the Northern Hemisphere, where the pairs diagonally opposite each other on the diagram swap around. This is an example of Northern Hemisphere bias which is common in articles on this topic.

In fact, our 'four season' idea is also biased: places within the latitudes 23° either side of the equator don't experience the same four seasons that we do. This is because their day lengths don't change enough to seriously change the weather. Instead, they tend to experience weather changes caused by warming and cooling oceans. Often, this causes a wet season and a dry season.

Here is a link to a Khan Academy lesson on this topic which includes an animated simulator.

The Moon

Earth is unusual in our Solar System because it is orbited by a moon whose mass is a big fraction (12%) that of the planet it orbits. Scientists think, for a range of reasons, that the Moon was formed in the early days of the Solar System by a collision between the proto-Earth and another protoplanet.

The famous US flag is crumpled, not rippling in the breeze. It swung like a pendulum for a few seconds after being erected but hung motionless until the gas from the departing lander once again disturbed it.

The orbit of the Moon and lunar phases

The Moon orbits the Earth at a distance of between 357,000 km and 407,000 km. At the closest point (perigee) the Moon is noticeably larger than usual in the sky. The Moon actually comes this close once every orbit, but we tend not to notice the larger size unless it coincides with a Full Moon,. We call such an extra-large Full Moon a 'super-Moon'.

The Moon always keeps the same face towards the Earth, although 'wobbles' in its orbit mean that we see a bit more than 50% of the Lunar surface over an extended period of time. This means the the lunar day/night cycle lasts for one Lunar Month. Daylight anywhere on the Moon's surface lasts fora bit over two Earth weeks, as does night.

The far side of the Moon has the same day/night cycle as the near side - the ideas that it is dark is false. The far side of the Moon is dark during a Full Moon and light during a New Moon.

The unlit portion of the Moon is very dark and difficult to see against the black of space. For this reason, the Moon appears to change shape as it changes position around the Earth. We call these shape changes phases.

The diagram above makes it clear why First Quarter has its name: the Moon has gone a quarter of the way around the Earth (shown by the red line of orbit). Many students are confused because they think it should be a 'half-Moon', not a quarter.

Lunar quarters last approximately a week. The Moon has been used by many cultures as a method of marking the time intervals that lie between a day and a year. This includes the tradition of the Catholic Church: Easter Sunday is the Sunday after the first Full Moon after the March equinox. This is why the date for Easter moves around from year to year.

Tides

If you were on the Moon's surface you would weigh about one-sixth what you do on Earth (your mass would stay the same). Gravitational force depends both on mass (the Moon is about 12% the mass of Earth) and distance. At a distance of 400,000 km away the Moon's gravity isn't detectable to our senses. It can be detected with instruments. It also causes effects on the atmosphere and the oceans and even on the continents. We call these effects tides.

Although you might think there should only be one bulge where the water is pulled towards the Moon, there are actually two. The reasons for this are quite complicated and we won't go into them here.

In reality, tides in a place like New Zealand don't follow the pattern above because the land gets in the way of the moving water. High tide tends to be a fixed number of hours after moonrise e.g. three hours on the Waitemata Harbour and six hours on the Manukau Harbour.

You can work out the time of moonrise from the phase e,g, New Moon rises at about dawn, First Quarter rises about noon and so on. Therefore you can work out a rough tide time if you know the phase of the Moon and the local 'tidal lag'.

High tide on successive days will be later by the same time span as moonrise is later - about 50 minutes. This means that the antipodal tide will be about 12 hours 25 minutes after the sublunar one.

Spring and neap tides

The Sun also exerts gravitational force on Earth, but a much weaker one because it is so much further away. If there was no Moon, we would still have tides but they would only be about 15% as strong as our lunar tides. They would also be the same time every day. These are called solar tides

If the solar high tide and the lunar high tide occur at the same time, the actual high tide you experience is larger because they add together. This is called a spring tide. It occurs when the Sun, the Moon and the Earth are in a straight line i.e. New Moon or Full Moon.

The opposite happens then the solar tide and the lunar tide are at 90° from each other, and the difference between high and low tide is at its smallest. This is called neap tide. It happens at First and Last Quarter.

Eclipses

The Moon's orbit is tilted, or inclined, with respect to the Earth's orbit around the Sun (the ecliptic). The angle of inclination is about 5°. This means that most of the time the Moon is either above or below the ecliptic and its shadow doesn't fall on the Earth.

Twice per orbit the Moon must pass through the plane of the ecliptic. Both the Moon's shadow and the shadow of the Earth both lie on this plane. If this happens when the Sun, the Moon and the Earth are in a straight line there will be an eclipse (which is where the ecliptic gets its name from).

During a New Moon the Moon's shadow can fall on the Earth. This is called a solar eclipse.

The umbra is where people see the whole of the solar disc covered by the Moon and there can be complete darkness. The penumbra is the region where people see only part of the Sun covered.

Looking from space, the umbra is a dark spot a few hundred kilometres across. The Earth rotates under it, so people in the 'path' of the umbra will be moved in and out of it quite quickly by the turning Earth. The period of total eclipse (totality) is quite short in any one place, but the actual period of there being totality during an eclipse can be hours.

The penumbra is much larger, so many more people see a partial eclipse. Most of these people will be carried through the Moon's shadow without hitting the 'bullseye' of the umbra.

The Earth's shadow is much larger then the Moon's and is easily capable of covering the whole Moon. Lunar eclipses happen at Full Moon.

The Moon could only pass through the penumbra, but sometimes passes through the umbra as well. If the whole Moon enters Earth's umbra it is termed a total lunar eclipse.

Total lunar eclipses can be marked by a strange phenomenon called a 'blood moon', where the Moon develops a reddish tinge at totality. This is caused by red light being refracted through Earth's atmosphere as shown in the diagram above.

Lunar eclipses are visible to everyone on the night side of the Earth. Solar eclipses, by contrast, are only visible to those in the umbra or penumbra. The penumbra is usually about the size of Australia and the umbra is a bit bigger than Fiji. This means that a person in a particular place will be much more likely to experience a lunar eclipse than a solar one, even though they occur about as often as each other.

Revision crossword

The changing starscape - the "Zodiac"

Stars are a long way away - the nearest star to the Sun is more than 200,000 times further away from Earth than the Sun. This makes the position of the stars seem to change fairly little as the Earth moves around the Sun. This is a bit like when you are driving on a country road - the close up trees move past quickly but the distant hills seem to stay still.

Our inability to see the different distances of stars causes us to see them as forming imaginary 2D patterns known as asterisms. Many cultures have turned these into 'pictures' with associated mythology. We call these pictures constellations. The "signs of the Zodiac" are such imaginary pictures made from stars along the ecliptic. These are shown below:

The pictures above are all shown in their Northern Hemisphere shapes - to us in New Zealand they are upside down, so the imaginary scenes they depict don't make much sense (to be honest, they don't really make sense even the right way up).

As the Earth moves in its orbit, the Sun appears to move through each of these twelve constellations. This happens when the constellation is 'across' the Sun from Earth:

For example, the picture above shows Earth in August. The Sun appears to be in the constellation of Leo. You can't see Leo at this time, because it is hidden behind the Sun. Instead, you would see Aquarius high in the night sky .

The dates for the Zodiacal star signs on a horoscope don't correspond exactly to the position of the Sun in the constellation because some constellations are much bigger than others, so astrologers have fiddled things to even up the times. Given that astrology is complete rubbish from a science point of vies, that hardly matters. Remember that astrology is the field associated with horoscopes and so on - it is completely mythical and has nothing to do with reality. Astronomy is the scientific study of stars.

Matauranga Iwa - Maori star knowledge

Maori were very familiar with the stars (iwa). Aotearoa has clear skies, no city lights (before European colonisation) and has no dangerous animals to stop you going out after dark.

Polynesian navigators had long used star lore as part of their skillset for navigating across the Pacific. Although individual stars were named and known, the concept of constellations as used in the Northern Hemisphere didn't really exist. A few asterisms were named.

Significant stars included Takurua (Sirius), Whanui (Vega), Te Kokota (Aldebaran). Some recognised asterisms included Taki-o-Autahi (Southern Cross), Te Kakau (Orion's belt) and Te Waka-o-Tama-rereti (the 'tail' of the Scorpion),

One of the most significant stellar features to Maori was Matariki. This is a 'stellar cluster' and is known to many cultures - for example, the Japanese call it Subaru and the Greeks knew it as the Pleiades.

Six or seven of these stars are visible to the unaided eye if you have good eyesight. The actual number of stars in the cluster is several hundred.

In Maori, Matariki means 'tiny eyes' or 'eyes of God'. The children of the earth mother Papatuanuku and sky father, Ranginui, were divided over whether to separate them and bring light and life to the earth. The eyes of Tawhirimatea have become many constellations, and among them live Matariki, the mother and her six daughters. They rise after the winter solstice and use their mana - their power - to help the weakened sun on his journey back south.

Matariki lies in the constellation we call Taurus. Therefore it is high in the evening sky in November and gradually moves towards the sunset as the Earth goes around the Sun through December to March. By mid-April it has disappeared below the horizon at sunset and is hidden behind the Sun during the day.

As Earth continues to move, Matariki starts to reappear, rising before the Sun in the morning sky. about mid June. This corresponds to the time of the winter solstice in New Zealand, so the season of Matariki marks the time when the days stop getting shorter and once again start lengthening towards summer. This is why Matariki is associated with renewal and growth.

The early Polynesian settlers would have been relatively unfamiliar with winter - it is likely that the first Maori in New Zealand came from somewhere in French Polynesia or the Cook Islands. Seasons there are not very prominent, with most crops being able to be planted and harvested year-round.

In Aotearoa, particularly in Te Wai Pounamu, the situation was very different. Cultural knowledge about the change of seasons became crucial to survival, because planting and harvesting had to be done at the right time or there wouldn't be enough food. This is a common feature of cultures in high latitudes. For example, the structure at Stonehenge in England is built in such a way as to mark the position of the Sun at the soltices.

Another such feature is Maeshowe in the Scottish islands. Here, the Sun shines directly up an underground passage at sunset on Winter Solstice:

Maeshow was the basis for the fictional Aslan's How in the Narnia stories.