measurable. Right now the Earth’s northern axis points toward Polaris, the Pole Star, which is how the star got that name in the first place. But it wasn’t always pointed that way, nor will it be. As the axis precesses, it points to a different part of the sky. Back in 2600 b.c. or so it pointed toward Thuban, the brightest star in the constellation Draco. In a.d. 14,000 or so it will point near the bright star Vega.
For astronomers, precession is a bit of a headache. To measure positions of astronomical objects, astronomers have mapped out the sky in a grid much like the way cartographers have mapped the surface of the Earth into latitude and longitude. The north and south poles on the sky correspond to those same poles on the Earth, but the sky’s north pole moves due to precession. Imagine trying to figure out directions on the Earth using north, south, east, and west if the north pole kept wandering around. You’d need to know just where the north pole was to know in which direction you needed to go.
Astronomers have the same problem on the sky. They must account for the precession of the Earth’s axis when they measure an object’s position. The change is small enough that most sky maps need to be updated only every 25 to 50 years. This is particularly important for telescopes like the Hubble Space Telescope, which must point with incredible accuracy. If the precession is not included in the calculation of an object’s position, the object might not even be in the telescope’s field of view.
Precession has an immediate impact on astronomers but a much slower one on the seasons. Right now, the Earth’s north axis points toward the Sun in June. But due to precession, 13,000 years from now — half a precession cycle — the Earth’s north pole will be pointed
Remember, too, that we are closest to the Sun on our elliptical orbit in January. So half a cycle from now the northern hemisphere of the Earth will experience summer when the Earth is closest to the Sun, amplifying the heat. It’ll also be winter when we’re farther from the Sun, amplifying the cold. Seasons will be more severe. In the southern hemisphere, the seasons will be even milder than they are now, since they’ll have summer when we are farther from the Sun and winter when we are closer.
This works the other way, too: 13,000 years in the past, the seasons were reversed. Summers were hotter and winters were colder in the northern hemisphere. Climatologists have used that fact to show that things might have been profoundly different back then. The slow change in the direction of the Earth’s axis might have even been the cause of the Sahara becoming a desert! On a year-by-year basis precession is barely noticeable, but over centuries and millennia even small changes add up. Nature is usually brutal and swift, but it can also display remarkable subtlety. It just depends on your slant.
6.
Phase the Nation: The Moon’s Changing Face
I never know whether to be surprised at the fact that, of the all the topics touched by bad astronomy, the Moon has the longest tally.
I’m surprised because the Moon is probably the most obvious of all astronomical objects. Some might argue the Sun is, but you can never really look right at the Sun. It’s always in the corner of your eye but never fully in it.
The Moon is a different story. When the night is dark, and even the crickets have gone to sleep, the full Moon shines down in blazing contrast to the black sky. Even as the thinnest of crescents the Moon commands attention, hanging low in the west after sunset. Whether high in the sky or low near the horizon, it dominates the night.
So it surprises me that there is so much misunderstanding about the Moon. I would think that since it’s such a common sight, it would be the best understood.
But perhaps that’s naive. After all, the more we know about something, the more room there is to
Why does the Moon look bigger near the horizon than when it’s overhead? Why does it have phases? How does it cause tides? How can it be up during the day? Why does it show only one face to the Earth? Which part is the dark side?
These topics all have some pretty hefty bad astronomy associated with them, and I promise we’ll get to all of them. But first things first. The most obvious aspect of the Moon is that it
A lot of people think it’s due to the shadow of the Earth falling on the Moon. The Moon is a big sphere, so when it’s almost all the way in the Earth’s shadow, the thought goes, the Moon is a crescent. When it’s fully out of the shadow, it’s full.
That’s a clever idea, but incorrect. The Sun is the major source of light in the solar system. That means the Earth’s shadow always points away from the Sun. That, in turn, means the Moon can only be in the Earth’s shadow when it’s on the opposite side of the sky from the Sun. But the Moon can’t always be in Earth’s shadow, especially when it’s near the Sun in the sky. We also know that when the Moon gets directly between the Earth and the Sun we get a total solar eclipse. That’s a pretty rare event, yet the Moon’s phase changes every night. Clearly, the Earth-shadow theory cannot be correct, and something else must be going on.
So what do we know about the Moon? Well, it’s a big ball, and it orbits the Earth once a month. Actually, the word “month” is derived from the same root as the word “Moon.” The phases change as the Moon goes around us, which is a clear indication that they must have something to do with the orbit. In science, it’s usually best to take stock with
New Moon marks the beginning of the lunar cycle of phases, which is why it’s called new. When the Moon is new, it’s completely dark. This happens when it’s near the Sun in the sky. Since the Sun is so bright and the Moon is dark, the new Moon can be very difficult to see. The Islamic month, for example, begins at the time the very earliest new Moon can be spotted, and so the followers of Islam keep very careful records and have keen-eyed observers ready to see it as early as possible.
First quarter is when the Moon is half lit, confusingly enough. It’s called first quarter because the Moon is lit like this when it’s one-quarter of the way around the Earth from the Sun, roughly one week after new Moon. For people in the northern hemisphere of the Earth, this means the right-hand side of the Moon — the side facing the Sun — is lit and the left-hand side is dark. For people in the southern hemisphere the reverse is true, since, to the view of people in the north, people in the south are upside-down.
A week later, the Moon is full. The whole disk is evenly illuminated. When the Moon is full it’s opposite the Sun in the sky, and it rises when the Sun sets.
A week after that, the Moon is at third quarter. Just like when it’s at first quarter, the Moon is half lit, and, also like first quarter, the half facing the Sun is lit. This time, though, it’s the other half that’s lit. From the northern hemisphere, the
Finally, a week later, the Moon is new again, and the cycle repeats. There are also names for the phases of the Moon when it’s between these four major ones. As more of the visible part of the Moon becomes lit, we say it is waxing. When the Moon is between new and first quarter, it’s still crescent shaped but it’s getting fatter, approaching half full. We say the Moon is now a waxing crescent. After it’s half full and approaching full, it’s in the gibbous phase, or, more accurately, waxing gibbous. After it’s full, it starts getting smaller. This is called waning. The Moon is waning gibbous from full to third quarter, and a waning crescent from third quarter to new.
So now we have names for all those shapes. The question remains, why does the Moon go through phases? Now that we’ve looked at them, we’re closer to figuring that out. However, there’s one more thing I want you to do. Go get a ping-pong ball or a baseball. Don’t have one? That’s okay, you can use your imagination.
Imagine that you are holding a white styrofoam ball. This is our model of the Moon. You will be the Earth
