See Venus pass Mars in the evening sky this week - WRAL.com

By Tony Rice, NASA Ambassador

Venus and Mars will be separated by just half a degree tonight and will remain within one degree throughout the week.

To see the pair, look to the west after dark. Venus will appear first in the twilight. This Earth-sized neighbor is only about 130 million miles away and light reflects very well off the clouds that cover the planet causing it to shine very brightly.

You'll have to wait until 9:15-9:30 for the Sun to be sufficiently below the horizon for Mars to appear. Even at another 100 million miles away and not as reflective, you should be able to make out a slight orange color to the planet in our clear skies this week.

How bright a star, planet, moon or passing satellites appears in our sky is controlled by several factors.

• reflectivity: Venus' cloud tops and the solar panels on the International Space Station are very reflective, the Moon's surface is less reflective
• distance: light intensity drops very fast with distance. A planet twice as far away appears four times dimmer.
• size: hundreds of times more sunlight reflected from massive Jupiter than relatively tiny Mercury
• phase: not only our Moon but, being closer to the Sun, Mercury and Venus display a full set of phases from crescent to full.

Math behind the brightness difference

Amaze your friends and neighbors not just with your ability to identify these planets but with your math skills too!

Astronomers measure brightness using a visual magnitude scale. Lower numbers are brighter, a lot brighter. A magnitude difference of 1 equates to an increase of 2.5 times in brightness, a 5 magnitude difference is 100 times brighter. The difference in brightness can be aproximated using the formula:

Δ_brightness = 100 ^{(magnitude₁-magnitude₂)/5}

At 9:30 p.m. tonight, Venus will be shining at magnitude -3.87, but the 11 layers of atmosphere you are looking through that close to the horizon reduces that magnitude to -2.87.  Mars will be shining at 1.84, reduced to 3.08.

That difference of nearly 6 in magnitude works out to Venus being nearly 240 times brighter than Mars.

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2021-07-12 15:23:04Z
CBMiT2h0dHBzOi8vd3d3LndyYWwuY29tL3NlZS12ZW51cy1wYXNzLW1hcnMtaW4tdGhlLWV2ZW5pbmctc2t5LXRoaXMtd2Vlay8xOTc2ODYyOC_SAVtodHRwczovL3d3dy53cmFsLmNvbS9zZWUtdmVudXMtcGFzcy1tYXJzLWluLXRoZS1ldmVuaW5nLXNreS10aGlzLXdlZWsvMTk3Njg2MjgvP3ZlcnNpb249YW1w