Big Ideas in Astronomy | Big Idea 3

Big Idea 3 - The night sky is rich and dynamic


We can see several thousands of stars with our eyes on a clear and dark night

When we gaze up into the night sky, far away from the light pollution of cities and during a New Moon or when the Moon is not in the sky, we can see roughly 4000 stars with the naked eye. All the stars we see with the unaided eye belong to our galaxy. Although there are billions of stars in other galaxies and trillions of galaxies in the observable Universe, those stars are too far away and hence much too faint for our eyes to distinguish them as individual points of light. Depending on our location on Earth and on the time of observations, our Solar System’s five brightest planets, the band of the Milky Way, two satellite galaxies of the Milky Way (the Large and Small Magellanic Clouds), and the Andromeda Galaxy (a big spiral galaxy) are also visible to the naked eye.


The night sky can help us to orient on Earth and navigate

Looking up at the night sky allows us to find the cardinal directions. In the Northern Hemisphere, the easiest way to find North is to look for the star Polaris, also known as the North Star, which is very close to the celestial North pole. The easiest way of finding Polaris is through the constellations of Ursa Major and Ursa Minor. In the Southern Hemisphere, the star Sigma Octantis, which is the closest star to the celestial South Pole is not easily visible. However, one quick method to find South is to use the constellation Crux and the two brightest stars in the constellation Centaurus.


The rotation axis of Earth wobbles (precesses) over thousands of years

As Earth rotates on its axis, it moves like a rotating spinning top. The direction of its axis of rotation changes in a slow precession with a period of about 26,000 years. This movement causes the axis to point in different directions over time and, as a consequence, the celestial North and South poles slowly change position over time. As an example, Polaris will eventually cease to indicate the North direction, although another star might, depending on the direction of Earth’s axis at the time. Although there is currently no bright star near the celestial South pole, in the future we will have a proper “South Star”!


Only a few celestial bodies are bright enough to be seen with the unaided eye when the Sun is above the horizon

Most of the objects in the night sky are too dim to be observed against the bright, sunlit sky. A similar effect happens at night in cities, where, due to light pollution, we can see only a small fraction of the stars due to the brightening of the sky by artificial lighting. Only a few celestial bodies are bright enough to be seen with the unaided eye when the Sun is above the horizon. Depending on its phase, it is possible to see the Moon during the day. At certain times, Venus can be observed in the morning (“Morning star”), in the evening (“Evening star”), and if you know where to look, Venus is also visible in the midday sky. Very rarely, a particularly bright comet might be visible during the daytime.


Celestial objects rise in the East and set in the West due to Earth’s rotation

Due to Earth’s rotation around its axis from West to East, an observer on the surface sees the whole sky move in the opposite direction, from East to West, seemingly rotating around our planet. This apparent movement of the sky around Earth is called Diurnal movement. This is the reason why we see celestial bodies rise above the eastern half of the horizon, and set below the western half.


Stars twinkle due to our atmosphere

As a star’s light enters our atmosphere and travels through its different layers, it constantly changes direction due to changing refraction in layers with different temperature and density. As a consequence, the brightness of a star’s light, and the direction from which it reaches us here on Earth, are constantly changing. For this reason, for an observer on Earth, the stars appear to twinkle. For planets, the effect is much less apparent (or perceptible). The reason is that planets can actually be seen as small disks (readily discernible using binoculars, for example). Stars, on the other hand, appear to us as tiny points of light, and because all the light is coming from a single point, it is highly susceptible to changes in refraction.


Millions of meteoroids enter Earth’s atmosphere daily

A meteoroid is a small rocky or metallic object ranging from the size of a grain of sand to one metre. When it enters Earth’s atmosphere it is heated by ram pressure, which creates a streak of light in the night sky. This phenomenon is called a meteor (or a shooting or falling star). When a meteoroid survives its passage through Earth’s atmosphere and lands on the surface, it is called a meteorite. Although millions of meteors occur in Earth’s atmosphere daily, most of the meteoroids that they originate from are burned to gas and dust before reaching the ground.