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Moon, Saturn on September 26

Tonight – September 26, 2017 – look in the southwest sky as soon as darkness falls to spot the waxing crescent moon pairing up with the planet Saturn. The moon and Saturn will sink downward as evening deepens, to set beneath the horizon by mid-evening. In other words, your best view of the moon and Saturn happens at nightfall, while the twosome beams highest in the sky. Nightfall presents the best time to view Saturn’s gorgeous rings in a telescope.

Saturn, the 6th planet outward from the sun, has the lowest density of all the planets in the solar system. Saturn is the only planet that’s less dense than water, having about 70% its density.

Image via NASA. Saturn is one of the four outer planets, approximately to scale. From bottom to top: Jupiter, Saturn, Uranus and Neptune.

For this reason, it’s often said that Saturn would float in water – that is, if you could find an ocean of water that’s wide enough and deep enough to contain it. On the other hand, it’s also been argued that Saturn would break apart and its core would sink in an ocean of this size.

By the way, do you know which planet of our solar system is the most dense? Yes, it’s our planet Earth, with over 5 times the density of water. But all four inner terrestrial planets – the rocky words orbiting the sun inside the asteroid belt – are quite dense: Mercury, Venus, Earth and Mars. The four outer planets – the gas and ice giants orbiting the sun outside the asteroid belt – are less dense, although much larger than the inner planets: Jupiter, Saturn, Uranus and Neptune.

Image via NASA. The four inner terrestrial planets from left to right: Mercury, Venus, Earth and Mars. Sizes to scale.

Mean density of solar system planets (density of water = 1.000 kilogram per cubic meter)

Inner planets:

Mercury: 5.427
Venus: 5.243
Earth: 5.514
Moon: 3.340
Mars: 3.933

Outer planets:

Jupiter: 1.326
Saturn: 0.687
Uranus: 1.271
Neptune: 1.638

Source: Planetary fact sheet

Around the world tonight – on September 26, 2017 – look for the waxing crescent moon to pair up with Saturn, the solar system’s least-dense planet.

Bruce McClure