What does the night sky look like from the equator?

All the stars in the universe move, from the smallest ones like asteroids and comets, to galaxies.

However, except for Solar System objects, they are so far away that we cannot see their motions with the naked eye on human time scales.

The changes that we see in the position of the constellations and the stars throughout the night, or between one night and another, are due to the movements of the Earth, both the rotation around itself and the translation around of the sun.

To know which stars we are going to see each night, in addition to the movements of the Earth, we have to take into account the latitude of the place from which we observe, because the portion of the sky that we can see varies from one latitude to another.

The most extreme cases are the North Pole and the South Pole.

From the Arctic you only see the northern half of the sky, which is what we call the Northern Hemisphere, and from Antarctica, you only see the Southern Hemisphere.

If we stand at the North Pole and start walking towards the equator, we will see more and more of the southern hemisphere.

For example, from Spain you can see Scorpio and Sagittarius, which are southern constellations.

When we reach the equator, we will be able to see all the constellations.

That does not mean that we see them all at once.

As we all know, stars can only be seen at night.

That means that we only see the stars that are on the side of the Earth opposite the Sun, which is where we look during our night.

But if we stand on the equator and observe the stars every night at the same time, throughout the year we will see all the stars because the Earth will have made a complete revolution around the Sun.

Although all the constellations can be seen from the equator, some stars are seen very low.

That is the case of the Polar star, which is the one that marks the north and that would be seen on the horizon line, so in practice it could not be observed.

And the same thing happens with the star Sigma of the South Octant that is located at the South Pole.

However, the Southern Cross, which is a constellation that has been widely used by navigators to navigate, is better seen because it is not located exactly at the South Pole.

We know that due to the rotation of the Earth all the stars rise in the east and set in the west every day, traveling an arc in the sky.

If we are in the northern hemisphere, that arc is seen in the south direction.

On the contrary, if we are in the southern hemisphere, the arc is seen in the north direction.

At the equator, that arc is almost vertical.

It is not quite vertical because the Earth's axis is tilted 23.5° to the ecliptic (the plane that defines the Earth's movement around the Sun), but it is very close.

We could say that, at the equator, the Sun and the Moon literally pass over our heads.

In addition, the duration of the day is always twelve hours, the same as that of the night and regardless of the season of the year.

The way we see the phases of the Moon also depends on where on Earth we observe them from.

In the northern hemisphere, when the Moon is in the first quarter, the illuminated area has a shape similar to a D, while in the southern hemisphere, we would see it in the shape of a C. At the equator, when the Moon is in the first quarter, it has the shape of an inverted U when it appears in the east and a normal U when it appears in the west.

When it is in the last quarter it would be the other way around, it appears in the east in the shape of a U and sets in the west in the shape of an inverted U.

You may also be interested to know that in the 18th century a very important French-Spanish geodetic expedition was carried out to the equator.

The objective of this expedition was to determine the shape of the Earth and, on behalf of Spain, the sailors and scientists Jorge Juan and Antonio de Ulloa participated.

That expedition managed to demonstrate that the Earth is flattened at the poles.

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