Thursday, March 4, 2021

Reserve the prospective

From Earth there are five planets in the Solar System that are bright enough to be seen with the naked eye: Mercury, Venus, Mars, Jupiter and Saturn.

Now let's try to reverse our perspective: suppose we move from planet to planet, seeing from case to case what the Earth looks like.

For example, let's imagine that we are in orbit around Venus. In this situation we could not observe from its surface as it is surrounded by a thick atmosphere that prevents any observation. From this position the Earth is the brightest celestial body in the sky after the Sun: in fact, in opposition it would appear to have an apparent magnitude of -6.6.

Let us now move on to the other two planets of the inner Solar System. The apparent magnitude of the Earth increases slightly, but it still remains one of the brightest objects in the sky.

For example, if observed from Mercury, the Earth appears of apparent magnitude -5, while from Mars its magnitude rises to -2.5.

Moving instead towards the planets of the outer Solar System, the Earth shrinks more and more, until it becomes a small point like many others and then disappears from view and becomes invisible. This is shown in this iconic image taken by the Cassini probe from the orbit around Saturn: the Earth from this position is in fact nothing more than a point more than a billion km away that is lost between the rings of the planet.

For all our abilities, humans are puny creatures on the scale of space. The Universe we exist in is just too vast and beyond our intuitive contemplation. As we start the new year with wishes and resolutions, here is a size comparison of our Earth to things in the Universe. It should help us remember just how insignificant we really are.

Credit: NASA, JPL, SSI.

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