Posted by DrJeff on September 28th, 2009

Filed under 1.4. Teachable Moments in the News, 5. Space Science, 5.1. Our Solar System, 5.1.3. Planets, 6. Cool Spacecraft

Copyright 2009 | About this blog

Given the BotU Special Post for the MESSENGER Flyby, I thought this would be a great supplemental post.

Some Cool Background

Imagine you’re looking at a bug flying around an outdoor light bulb at night. Let’s say you’re looking at it from a distance which is always greater than the distance the bug is from the bulb. Wow. It’s a really interesting bug you’ve never seen before, and you want to share the experience with a friend, or (in my case) your son or daughter. I might say “Hey Jordi! Check out this really cool bug!” He’d say, “Daddy, where??” Ok, now I’ve got to tell him where to look. What would you say? How about: “over there, near that light bulb.”

Well this is EXACTLY the situation with the planet Mercury for earthbound observers. Mercury is orbiting the Sun and we’re looking at it from Earth, which is at a greater distance from the Sun than Mercury is from the Sun.

So if you want to see Mercury in your sky, you need to look … near the Sun. Anybody see a problem with that? The Sun is a pretty high wattage light bulb, and if it’s up in the sky, you’re not going to see Mercury or the stars for that matter. They’re up there with the Sun but their light is absolutely swamped by the sunlight illuminating our atmosphere.

Just so you know, the Sun light bulb is General Electric model #big01bertha, and its wattage is 383,900,000,000,000,000,000,000,000 Watts. It is also guaranteed for another nearly 5 billion years of operation. Handle with care.

Now back to the bug. So Jordi takes a look in the direction of the light bulb, and he sees the bug whenever it’s off to the side of the bulb. When the bug is in front of the bulb, it’s lost in the bulb’s light.

So that’s the secret to seeing Mercury in the sky! There are times when Mercury is far enough to the side of the Sun, so that it can be seen above the horizon—because the pesky Sun is below the horizon. The positions of Mercury and Earth in their orbits have to be just right so that we can see Mercury either BEFORE sunrise or AFTER sunset.

Now for the way cool part. It’s that time right now! While MESSENGER is flying by Mercury you can see the planet in the east before dawn!

How To See Mercury Before Sunrise September 29 Through October 1

The Graphic above is taken from This Week’s Planet Roundup in Alan MacRobert’s column This Week’s Sky at a Glance, at Sky & Telescope magazine. I just spoke to Alan to get some pointers for Mercury viewing to pass on to you. Here’s the poop:

• The graphic shows the situation at 40 degrees north latitude but it’s pretty much the same for the entire continental US (e.g., Bangor, Maine: 44 degrees; Miami, Florida: 25 degrees), and for the same latitudes around the globe.

• You need to look about 45 minutes before your local sunrise. Any closer to sunrise and the sky will already be too bright. So you need to get your local sunrise time. How? You can use the Sky & Telescope online Almanac or see if it is provided in your local paper.

• You need to look due east. How can you tell if you don’t know which way to look? Due east will be the brightest area of the sky on the eastern horizon, because Big Bertha (the Sun) is about to rise over there. If you’re one of those folks that likes to plan in advance, so you need to know the direction of “due east” right now, well here’s a plan. Go to Google maps, put in your address, or the address of the site from where you’ll be looking, select “Satellite” view, zoom in until you see a recognizable building or landmark, and note that NORTH is up on the map.

• Mercury will be low in the sky, only about 10 degrees above the horizon, so you need to view from a spot where due east is unobstructed by trees and buildings.

• Venus is going to really help you. It will be a bright star-like object, but it won’t be twinkling like the stars (planets don’t). You can see in the graphic at the top of the page that Mercury will be below Venus. Mercury will be far dimmer than Venus, but can be seen with the unaided eye. BINOCULARS will help.

When you look at Mercury, you might recal that YOU’RE LOOKING AT THE MESSENGER SPACECRAFT TOO. You’re looking at history, only from really really far away. So what—it’s still history!





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