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Starts With A Bang

Which Planet Appears The Largest From Earth?

Mars will never look as big as the full Moon. But it isn’t even the biggest planet.

From humanity’s perspective, the Sun and Moon always dominate Earth’s skies.

The Moon and Sun are both approximately half a degree in angular size, as viewed from Earth, with slight variations due to the elliptical nature of Earth’s and the Moon’s orbits. Yet even at minimum, the apogee Moon is a whopping 1764″ (arc-seconds) in size, far greater than the 66″ maximum achieved by the largest (Venus) of the planets.(EHSAN ROSTAMIZADEH OF ASTROBIN)

They outshine everything in terms of brightness, with superior appearances in terms of angular size: around 30′ (arc-minutes): half a degree.

On July 21st, 2018, Fernando Cabririzo took a mosaic of the Moon and a number of astronomical objects, using the same telescope and the same camera. You can see the snapshots of the true, relative size between the Moon and all the planets except Mercury, which was missed due to clouds near the horizon. The ISS and Pluto were also captured. (FERNANDO CABRERIZO (CENTRO ASTRONOMICO DE TIEDRA))

All the planets in our Solar System are only somewhat larger than the Moon, but much farther away.

The eight planets of the Solar System. Their relative actual sizes are shown to scale, but their distances are not. When both factors are correctly combined, we can learn what the apparent, angular size (and size range) of each planet is as viewed from Earth. (WIKIMEDIA COMMONS USER WP)

When they’re closest to Earth, they appear largest.

Orbits of the inner and outer planets, all obeying Kepler’s laws. When Earth and another planet pass one another in orbit, they reach closest approach, and appear large relative to one another. When they pass each other on opposite sides of the Sun, they appear smallest. Mars, followed by Venus, experience the largest relative variations between apparent perigee and apogee size. (NASA / JPL-CALTECH / R. HURT, MODIFIED BY E. SIEGEL)

But their angular sizes all vary as the planets orbit relative to one another.

The best way to see Mercury is from a large telescope, as dozens of stacked images (left, 1998, and center, 2007) in the infrared can reconstruct, or to actually go to Mercury and image it directly (right), as the Messenger mission did in 2009. The smallest planet in the Solar System, its proximity to Earth means it always appears larger than both Neptune and Uranus. (R. DANTOWITZ / S. TEARE / M. KOZUBAL)

Mercury, the smallest planet, varies from 4.5″ to 13″, where 1″ (arc-second) is 1/3600th of a degree.

Excellent narrow-angle camera views of the planet Neptune taken from Voyager 2 spacecraft. (Time Life Pictures/NASA/The LIFE Picture Collection/Getty Images)

Both Neptune (from 2.2″ to 2.4″) and Uranus (from 3.3″ to 4.1″) always appear even smaller.

Voyager 2 flew by both Uranus (R) and Neptune (L), and revealed the properties, colors, atmospheres, and ring systems of both worlds. Neptune always appears smaller, in angular size, than Uranus does as seen from Earth, even though it’s the slightly larger world. (NASA / VOYAGER 2)

Their much greater distances from Earth ensure that.

Frost, icecaps and clouds are some of the watery features that can be seen from a great distance on Mars. These features are not visible today, unfortunately, due to the massive dust storm raging on the red planet. Its angular size, which varies by more than a factor of 7, showcases the greatest differences between closest and most distant appearance of any planet in the Solar System. (NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))

Mars experiences the greatest relative variation in angular size, from 3.5″ to 25.1″.

One of the most spectacular mosaic images of Saturn taken by Cassini, this 2016 view shows the north pole, the rings, the planet’s shadow, and the nearly-fully illuminated face of our Solar System’s most visibly-ringed world. Saturn’s radius is approximately 60,000 km, but if you include the main rings, they extend out to 140,000 km. With the rings included, Saturn can appear even larger than Jupiter does, despite being twice as distant as the Solar System’s largest planet. (NASA/JPL/SPACE SCIENCE INSTITUTE)

Saturn, the second-largest planet, ranges from 14.5″ to 20.1″, but if you include its main rings, becomes enormous, spanning from 33.8″ to 46.9″.

A size comparison of Earth and Jupiter shows how much impressively larger the biggest gas giant in our Solar System is compared to Earth. But it isn’t always the largest planet, in terms of angular size, from our point of view. (NASA; BRIAN0918 AT ENGLISH WIKIPEDIA)

Jupiter is the largest planet, residing quite far from Earth.

Jupiter, as photographed in 2004 from Earth, occasionally has a dark circle on it, as one of its moons casts a shadow on its atmosphere. Here, Ganymede, the largest moon in the solar system and a world even larger than Mercury (but smaller than Mars), has its shadow fall on its Jovian parent. (Jamie Cooper/SSPL/Getty Images)

For angular size, it ranges between 29.8″ and 46.9″.

An infrared view of Venus’ night side, by the Akatsuki spacecraft. Its brightness is greater than that of any other planet as seen from Earth, and it approaches our world closer than any other planet does. When it is on the other side of the Sun, only a few other planets appear smaller. (ISAS, JAXA)

Venus, our sister planet, comes closest to Earth, ranging from 9.7″ to a whopping 66.0″.

Galileo’s original (1610) sketches of the phases of Venus were made very shortly after the telescope was first turned skywards. Astronomers with very good vision, even before the invention of the telescope, had claimed to see a crescent shape to Venus when it was closest to Earth. (WIKIMEDIA COMMONS USER FERNANDO DE GOROCICA)

Humans with exceptional vision, at Venus’ closest approach, can barely discern its crescent phase without a telescope.

Mostly Mute Monday tells the astronomical story of an object, event, class, or phenomenon in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.