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Since I’m on vacation, I’m reminded each evening about how different the sky gets when you travel. Take Boston to Hawaii. Yes, the Moon will appear differently. Got friends and family in different locations? Planning travel? Let’s explore whether we all see the Moon and Sun in the same way.
Actually, changing your longitude, your east-west location, alters nothing. It’s going south or north that creates strangeness. So going from New York to Rome won’t change the sky. But Minneapolis to Miami or Alaska to Arizona is a different story.
The Sunset and Moonset
You see it with your first sunset. From Europe, Canada, or the northern three quarters of the U.S., it takes the Moon three minutes to move its own width as it sets, and the same timeframe applies to the Sun since it’s the same apparent size. Both of those disks require three minutes to completely pass through the horizon, because both of them always glide into the western horizon diagonally.
But from the tropics, which includes Hawaii, the Sun and Moon each drop straight down like cannon balls, creating the world’s fastest sunsets. This near-vertical downward motion also explains why twilight is shortest when you vacation closer to the equator.
Yes, we all see the same phases of the Moon from Earth, no matter where we live in the world. This is because Moon phases (how much of the disk is illuminated) is caused by the positions of the Sun, Moon, and Earth. See your Moon Phase.
However, the Moon does not look the same because we observe the Moon from different angles on Earth. In the Northern Hemisphere, the illuminated part of the Moon travels from right to left; in the southern hemisphere, the sunlit part seem to travel from left to right. Again, this all depends on traveling north or south, i.e., your latitude, not your east-west longitude.
Also, from the tropics, you’ll sometimes see the Moon straight overhead, which never happens from Europe, Canada, or the U.S. Expect that zenith Moon to happen at midnight when it’s full, and during sunset when it’s a first quarter half-Moon.
Seeing the Green Flash at Sunset
I traveled to Thailand for my vacation. If you travel to any tropical location, such as Hawaii, this offers the increased chances to see sunset’s fabled Green Flash. Not that it’s necessarily hidden elsewhere. Californians with cliffside homes facing the Pacific have the one necessary requirement every evening—a perfectly unblocked horizon. But vacationers in the tropics are certainly more likely than, say, Vermonters to not have sunset obscured by hills or trees.
Here’s how you see a green flash:
Enjoy a lot of sunsets! You need to be able to see below the real horizon, for example across water. If it’s a cloudless day, with steady temperatures, the final tiny dot of the setting Sun that appears orange will suddenly turn a lovely emerald green, just for one to four seconds, and then it too will set. (Do not ever stare right at the sun; look just after the sun is below the horizon.)
Here’s what happens:
When you look at the setting Sun or anything at the horizon, you’re looking through 40 times more air than when you’re looking overhead. This exaggerates air’s tendency to bend each color of sunlight at a slightly different angle. So, a setting Sun is actually a series of differently colored Sun images setting one after the other. The red Sun sets first, followed immediately by the orange Sun, then yellow. Green is the last to go, because the Sun’s blue image has been thoroughly dispersed and scattered away by the long path to your eyes.
How often does this happen? I’ve carefully watched for the green flash every time conditions were right, and saw it around 25 times. I’d looked for it over 200 times, so if my experience is typical, it would seem to appear about 10% of the time.
All these travellers’ sky effects require no equipment, charts, or even unpolluted rural skies. They’re there reliably. You just have to get out of town. Which, at this time of year, usually means to the south!