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Parallax

Parallax is the difference between the angular position of two points. In other words, it’s the apparent movement of an object viewed along two different lines of sight. It can be measured by the angle between the two lines.

For instance, when you hold out your thumb and close one eye, then open it and close the opposite eye, your thumb appears to move even when you’re holding it still. This is an example of parallax. The object stays in the same place, but since the direction in which your eye is viewing your thumb is different (the line of sight), it appears to be displaced.

Horizontal parallax is the parallax objects show when they are low on the horizon.

Parallax in Altitude is the parallax objects show when they are at an altitude, represented mathematically by the equation:

Par. in alt. = Horizontal par * Cas(alt)

Parallax can be used to measure the distances of celestial objects. We don't measure distances and size of celestial objects by closing one eye and then the other, because they are so far away that it wouldn't be accurate. We have to use a larger scale. Since the earth orbits around the sun, different stars appear in different places throughout the year. We can use parallax by observing a celestial object twice – in the same place on earth, but at different times of the year. Usually we observe the object once in January and once in July, six months apart. Of course, even with new technology, using parallax to measure distances is not completely accurate and reliable.

This is the equation that scientists use to calculate distances to a star:



Inverse Square Law

The inverse square law is a law in physics stating that one physical quantity is proportional to the distance from the source of the physical quantity.

For example, imagine that there are two lights that are able to spread light in any direction with no limit. Imagine that both lights have the same brightness, but one is farther away than the other. Which one will appear to be brighter? the light that was closer would appear to be brighter.

This property can be stated mathematically with the formula

, where I is Intensity of Object and D is Distance to object

We can use stars’ intensities to find their relative distances from the Earth and from one another. However, this is assuming that two or more stars being compared all have the same brightness to begin with, so it is not the most reliable way to calculate distances. Scientists do not generally use this method anymore unless there is no other way to measure the distances of celestial objects. However, many people associate brighter stars with being closer to earth, even if they don't know about the Inverse Square Law.

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