Of Particular Significance

Tag: SolarSystem

How Far Could the Sun Possibly Be?

(This is the third post in a series, though it can be read independently; here are post #1 and post #2; and post #4 blows this one out of the water, so don’t miss it!)

Measuring the distance to the Sun is challenging, for reasons explained in my last post. Long ago, the Greek thinker Aristarchus proposed a geometric method, which involves estimating the Moon’s sunlit fraction on a certain date. Unfortunately, because the Sun is so far away, his approach isn’t powerful enough; Aristarchus himself underestimated the distance. [This last remained true for later astronomers before the 17th century, though they got closer to the truth, presumably by using more precise methods than you or I could easily apply. I doubt anyone truly found a maximum possible distance to the Sun just using geometry.] The best we can do, using Aristarchus’ method and our naked eyes, is determine a minimum possible distance to the Sun: a few million miles.

Figure 1: A simple application of Aristarchus’ method tells us that the minimum distance to the Sun is a few million miles (km), ruling out the red region. But the entire green region is still allowed.

Today we’ll see how to obtain a maximum distance to the Sun, using an approach suggested in the previous post: by measuring speeds. Specifically, we’ll make use of a speed that the ancient astronomers weren’t aware of: the speed of light, also known as the cosmic speed limit c. That’s 186,000 miles (300,000 km) per second, or 5.9 trillion miles (9.5 trillion km) per year. We’ll find the Sun’s distance is less than 12 billion miles… still much larger than its true distance, but a significant improvement on our starting point!

*Figure 2: By the end of this post, we’ll know a maximum possible distance for the Sun, too.*

(more…)

POSTED BY Matt Strassler

ON January 19, 2024

Earth Goes Around the Sun? What’s Your Best Evidence?

It’s commonly taught in school that the Earth orbits the Sun. So what? The unique strength of science is that it’s more than mere received wisdom from the past, taught to us by our elders. If some “fact” in science is really true, we can check it ourselves. Recently I’ve shown you how to verify, in just over a dozen steps, the basics of planetary astronomy; you can

confirm that the Earth’s a spinning (almost-)sphere,
estimate the size of the Earth and Moon and the distance between them,
show the Sun’s larger than the Earth and much further than the Moon, and that the stars are further still,
verify the other planets orbit the Sun and estimate their relative distances from the Sun and their orbital times,
infer a relation between these distances and times known as Kepler’s law, and show that a similar Kepler-type law works for objects orbiting the Earth,
and infer from these laws that the same gravity that makes ordinary objects fall creates an inward acceleration, one that follows an inverse square law, holding certain objects in orbit around the Earth and others in orbit around the Sun.

But important unanswered questions remain. Perhaps the most glaring is this: Does the Earth orbit the Sun, or is it the other way around? Or do they orbit each other around a central point? The Sun’s motion in the sky relative to the stars, which exhibits a yearly cycle, indicates (when combined with evidence that the stars are, on yearly time scales, fixed) that one of these three must be true, at least roughly. But which one is it?

We saw that the Earth satisfies Kepler’s law for objects orbiting the Sun; meanwhile the Sun does not satisfy the similar law for objects orbiting the Earth. This argues that Earth orbits the Sun due to the latter’s gravity, but the logic is circumstantial. Isn’t there something more direct, more obvious or intuitive, that we can appeal to?

I won’t count high-precision telescopic observations that can reveal tiny effects, such as stellar aberration, stellar parallax, and Doppler shifts in light from other stars. They’re great, but very tough for non-experts to verify. Isn’t there a simpler source of evidence for this very basic claim about nature — something we can personally check?

Your thoughts? Comments are open. [Be careful, when making suggestions, that you are not assuming that gravity is the dominant force between the Earth and the Sun. That’s something you have to prove. Are you sure there are no additional forces pinning the Earth in place, and/or keeping the Sun in motion around the Earth? What’s your evidence that they’re absent?]

POSTED BY Matt Strassler

ON April 28, 2022

What to Watch in the Sky This Week: Beauty in Motion

Why does the sight of the Moon draw our gaze and silence our voices? What is it about the planets, those exceptionally bright points of light that wander among the stars, that we instinctively find so beautiful? Is it perhaps that they make us dream of faraway, unreachable places? Is it that they are beacons in the night, nature’s candle flames, helping keep fear at bay, and offering us hope amid darkness? Or is it perhaps that they seem to float — we do love things that float, whether they be autumn leaves, balloons, clouds or birds — suspended in the sky, in apparent defiance of the force of gravity which keeps us pinned to the Earth?

This last thought offers a certain delicious irony… for in truth the planets and the Moon, in their procession above our heads, obey gravity’s dictates.

The next few days, weather permitting, will give us a chance to contemplate these questions. Our planet’s natural satellite, on its monthly trip around the Earth, will pass three of the brighter planets in the sky, creating one lovely spectacle after another. Of course, the Moon really passes nowhere near the planets, just as your outstretched hand, when it blocks your view of the Moon, is nowhere near the Moon itself. It is all a matter of perspective — of geometry, of cavernous spaces, of the play of light, and of the elegant choreography of the solar system. But this perspective is not something we sense easily. Our eyes can perceive no depth for objects so far away, and so our brains form a two-dimensional picture from the three dimensions of the universe, projecting the Moon, the planets and the stars, at extraordinary distances from one another, onto a psychologically flat black screen of the night sky. It takes great mental effort to see things as they are, and not as they appear. This, too, is worthy of contemplation.

First, on the night of February 22nd, just after sunset (don’t be late!), one of the most delicate of nocturnal sights awaits: Click here to read more…

POSTED BY Matt Strassler

ON February 21, 2012

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Tag: SolarSystem

How Far Could the Sun Possibly Be?

POSTED BY Matt Strassler

Earth Goes Around the Sun? What’s Your Best Evidence?

POSTED BY Matt Strassler

What to Watch in the Sky This Week: Beauty in Motion

POSTED BY Matt Strassler

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