How to Measure Distances in the Night Sky

Sometimes, the apparent distance between two celestial objects the distance we can actually see on the sky is indicated in terms of angular degrees. But these descriptions can seem like a foreign language to folks who don't pore over star charts every day, so here's a handy primer.

If we measured the distance around the circle of the entire horizon from north all the way around through east and south and west and back to north again that would equal 360-degrees.

From the horizon to the point directly overhead (the zenith) would equal 90-degrees; from one horizon point through the zenith and continuing across to the opposite side of the sky would measure 180-degrees.

You can also use your clenched fist as a sextant to measure the height of the moon, star, or planet above the horizon. Your clenched fist, correctly held, will roughly measure off 10-degrees. So you can use your fist to make a reasonable guesstimate of degrees either horizontally or vertically.

The stars themselves can serve as handy yardsticks in the sky. The famous belt of Orion, for instance, measures 3-degrees and the twin stars of Gemini (Pollux and Castor) are 4-degrees apart.

This week, the constellation of Leo, the Lion conveniently stands high above the southern horizon at dusk. The backward-question mark pattern of stars popularly known as the Sickle forms the Lion's head and measures 14-degrees from top to bottom.

The distance between Leo's two brightest stars, Regulus (at the bottom of the Sickle) and Denebola (which marks the tip of the Lion's tail) is 24-degrees.

Close Encounters

Occasionally two planets, or a star and planet will appear exceptionally close together in the sky; their distance being less than a degree apart.

In such unusual cases, we would measure the distance between the two objects in tenths of a degree or in more extreme cases, in arc minutes. One degree, for instance, is equal to 60 arc minutes. One half of a degree, which is the average apparent size of the moon is equal to 30 arc minutes.

In the Big Dipper (or the Plough if you are reading this in the United Kingdom) the star at the bend of the handle is Mizar which appears to have a fainter companion about one-fifth as bright known as Alcor.

Mizar and Alcor together are sometimes called the "Horse and Rider," and the ability to resolve the two stars with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars. They are separated by just 0.2-degree or 12 arc minutes check them out tonight for yourself. Indeed, two bright planets or a bright planet and bright star that are separated by less than this distance can make for a stunning visual spectacle.

I did some checking and found some very close conjunctions coming up during the next 20-years in which two objects, either two bright planets or a bright star and a planet, will appear within a distance of less than 12-arc seconds. I've listed five such cases in the table below.

        DATE                         OBJECTS                 SEPARATION         VIEWING TIME

Historians and biblical scholars no doubt will have a special interest in viewing the Venus/Jupiter conjunction in August 2016, since some theorize that a similarly close approach of these two bright planets in the eastern twilight sky in August of 3 BC might have been what the Magi later identified to King Herod as the legendary Star of Bethlehem.

Even more intriguing is the close pairing off of Jupiter and Saturn on Dec. 21, 2020.

Conjunctions between Jupiter and Saturn occur on average once about every 20 years. But the 2020 case will be unusual in that rarely do these two planets ever appear to come so close to each other. Have you ever wanted to see Jupiter and its retinue of bright satellites passing Saturn and its famous ring system together in the same high-power field of a telescope?

You'll be able to do so on this night! In fact, the last time they were this closely spaced was in July 1623 and they will not appear so close together again until March 2080. Mark your calendars!

Our Deceptive Moon

Because it measures one-half degree in apparent size, some might think that the moon can be used to measure off angular distances, but this is far from the case.

For one thing, there is the famous "moon illusion" in that our natural satellite seems to appear much larger in size when it lies close to the horizon. Indeed, a rising moon can sometimes appear absolutely enormous, yet an hour or two later it will appear to have shrunk to a considerably smaller size. This strange effect has intrigued artists and puzzled psychologists for many years and has perplexed people since earliest historical times, such as Aristotle.

No one seems to know exactly why this happens, though the most popular explanation is that the moon illusion is an optical illusion related to the Ponzo illusion in that the human mind judges an object's size based on its background with foreground objects such as trees and houses tricking our brain into thinking the moon is much bigger than it really is.

But even when it appears high in the sky, the moon looks "too big" to be one-half degree in width. And this illusion is not confined to the real sky, but is even evident in the "pretend universe" of a planetarium.

When the very first projectors were designed and made to project the moon's image of one-half degree on the planetarium dome as it appears in the real sky, it was found that it appeared too small to be realistic although it was in actuality the correct angular size with respect to the background sky. [Full moon photos.]

To rectify this problem, engineers doubled the size of the projected moon image to one-degree, which presents a much more realistic appearance; one of the few places where accuracy was sacrificed for the sake of realism.

Similarly, the real moon appears much larger against the real sky. Try this mental experiment some night when you can see both the Big Dipper and moon in the sky at the same time.

First look at Dubhe and Merak, the two stars that are commonly used to point toward Polaris, the North Star. Now, looking at the moon and then back to Dubhe and Merak, try to estimate how many moons you could fit between the two stars.

Keep in mind that those two stars are separated by 5 1/2 degrees. And as we've already noted, the moon itself appears a half-degree in width.

That means you should easily be able to squeeze no fewer than eleven moons between the two stars. It is a fact that is very difficult to accept. Maybe four moons can fit in the space between the stars; five at the very most.

But 11? The night sky is full of surprises

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.