Two Bright Planets

I finished work at 6 pm today. At that time, the sky was clear, the sun is already 6-12 degrees below the horizon and Venus and Jupiter were shining brightly. I look at both planets, located in opposite sides of the horizon, and they both appear so bright. I wondered why, and came up with an answer.

Venus is many times closer than Jupiter, but they look similar in brightness. The reason why is because Jupiter is bigger and we can see more of it. Jupiter’s mass and radius is 1.89E27 kg (317.8 Earth Masses), and 69,911 km, whereas Venus is only 4.867E24 (0.815 Earth Masses), and 6,052 km respectively. Clearly, Jupiter is the larger planet, but (at closest point) is 588 million km away. Venus is the smaller planet, but is only (at closest point) 38 million km away. This means that they look similar in brightness when compared to each other.

Here is another example:

The sun is 1.9891 × 1030 kg, and 695,500 km in mass and radius respectively. The Moon is 7.34767309 × 1022 kg large and 1,737.5 km in mass and radius respectively. The sun is (clearly) the larger of the two, but when you put them on top of each other, such as during a Solar Eclipse, they look very similar in size.

For both cases, their actual distances and diameters vary greatly, but their angular diameter is similar. The angular diameter is the diameter of an object from a certain frame of reference. For example, the Moon is 31.075 arcminutes across, and the sun is 32 arcminutes across. Very similar in angular size, but greatly varied across the cosmic distances.

The next time you see Venus and Jupiter in the sky or a solar eclipse, Look at how different they are, but how similar they look. It’s will surprise you.


Keep looking up. You never know what you will find up there.



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