A ‘handy’ identification tool

Easy guide to using your hands to figure out celestial objects

When it comes to finding objects in the night sky, astronomers measure the distances between celestial objects, or between a celestial object and the horizon, based on the angle the object(s) makes with an observer or observational point on Earth.

This is referred to as "angular distances" or "angular separations’’ and is expressed in degrees.

The sky above us is a 180-degree half-dome (the other half is below the horizon to total 360 degrees). The half-arc from horizon to zenith (the point in the sky directly overhead) is 90 degrees.

Degrees are further subdivided into arc-minutes (1-60 of a degree) and arc-seconds (1-60 of an arc-minute); distances between very close celestial objects are typically expressed in arc-minutes and arc-seconds.

This system of degrees and their subdivisions is known as the “sexagesimal system” and is similar to the divisions of an hour into 60 minutes, and a minute into 60 seconds.

HOW ARE DEGREES USED?

Astronomers use degrees to measure angular distances the same way geographers measure latitude and longitude on the Earth’s surface. A celestial object’s altitude in the sky corresponds to a geographical latitude reference on Earth’s globe and the object’s azimuth (its position along the horizon from due north) corresponds to a longitudinal position.

You’re most likely to come across angular distances when referencing a star map or one of the numerous astronomical apps. The position of the celestial object (e.g., a planet) is typically stated in degrees above the horizon or a certain number of degrees (in a given direction) from another more prominent object (e.g., a faint galaxy near a bright star).

For example, on a particular night, Venus will become visible x number of degrees above the western horizon at a given time.

While this is easy enough when the object is very bright and easily spotted (Venus or Jupiter), it’s not always as easy when it’s one of the more distant and fainter planets (Uranus or Neptune), is an open star cluster (e.g. the Beehive Cluster in Cancer - the Crab) or a distant galaxy (like the Andromeda Galaxy in Andromeda - the Queen) that’s just at the edge of naked-eye visibility, even under a clear sky.

How, then, when our star chart shows that object X is 25 degrees above the southeast horizon at 9 p.m. tonight, can we find that object with enough certainty to be sure it’s the one we’re looking for?

USE WHAT’S AT HAND

One of the handiest and well-known techniques for determining angular distances in the night sky is using our hands and fingers.

This technique can be used by young and old alike, as the size of your hand is proportional to the length of your arm.

While hand sizes vary between people, so exact measurements may vary slightly, the following method is reasonably accurate enough for most people to use:

1. Hold your hand up in front of you (palm facing away from you, fingers and thumb pointing upwards) at arm’s length; keep your hand at arm’s distance at all times.

2. Make a fist while facing your clenched fingers away from you (as if you were going to knock on a door). The distance between the outer edge of your index finger and the outer edge of your little finger is approximately 10 degrees of angular distance in the sky.

3. Raise only your little (pinky) finger; the width across the tip of your little finger equals one degree of angular distance.

4. After lowering your little finger, raise your three middle fingers (it helps if you hold your little finger down with your thumb); the distance across the tips of your three middle fingers, from outer edge to outer edge, measures five degrees.

5. Holding your middle and ring fingers down with your thumb, extend your index finger and little finger upwards as wide as possible; the distance between the tip of your index finger and the tip of your little finger is 15 degrees.

6. Opening your entire hand as wide as possible will measure about 25 degrees from the tip of your extended little finger to the tip of your extended thumb.

Some people have much greater flexibility in their thumbs than others, and may be able to extend their thumbs further, resulting in a bit of a larger measurement of angular distance. By turning your extended hand on its side, you can use your hand and fingers to measure the angular distance of a celestial object (e.g., a star) above the horizon.

TRY IT OUT

To get an idea of how much angular distance your hand and fingers cover, go out on a clear night, look to the north for the Big Dipper in the constellation of Ursa Major the Great Bear, and compare the angular distances of your hand and fingers with the known angular distances between the stars in the Dipper.

The four stars that constitute the Big Dipper’s bowl are: Dubhe (upper right corner), Merak (lower right corner), Phecda (lower left corner), and Megrez (upper left corner).

The three stars in the Dipper’s handle are: Alioth (closest to the bowl), Mizar (the mid-handle star), and Alkaid (the handle’s end star).

From Dubha to Merak measures five degrees; from Dubhe to Megrz 10 degrees; from Dubhe to Alioth 15 degrees; from Dubhe to Mizar 20 degrees; and from Dubhe to Alkeid (the width of the Big Dipper) 25 degrees.

Incidentally, the angular distance between Dubhe and Polaris (the North or Pole Star) is approximately 30 degrees.

By using your hand and fingers to locate a star or planet above the horizon, or to determine the distance between two celestial objects, you always have, at your immediate disposal, the means to quickly determine the answer for yourself.

It’s also an easy and fun way to get young people interested in learning about the night sky; my granddaughter Scarlet can now measure her way around the night sky like a pro.

THIS WEEK’S SKY

Venus (magnitude -4.2, in Gemini - the Twins) is visible as darkness falls, 28 degrees above the western horizon around 9:15 p.m., before it drops towards the horizon and sets about 12:20 a.m. On June 4, Venus is at its greatest elongation east of the sun.

Mars (magnitude +1.5, in Cancer - the Crab) is visible 27 degrees above the western horizon, to the upper left of Venus, by 9:55 p.m., before it, too, dips to the horizon and sets by 12:50 a.m.

Saturn (magnitude +1.0, in Aquarius - the Waterbearer), rises in the southeast shortly after 2 a.m., reaching 22 degrees above the horizon before fading into the dawn’s first glow around 4:35 a.m.

Jupiter (magnitude -2.1, in Aries - the Ram) rises in the east around 3:55 a.m., reaching 10 degrees above the horizon before fading into the approaching dawn around 5 a.m.

Despite reaching its greatest elongation west of the sun May 29, Mercury is not observable this week, as it is no higher than zero degrees above the eastern horizon at dawn May 29, and no higher than five degrees above the eastern horizon at dawn by June 5.

ABOUT THE MOON

The full moon on June 4 was known by many names by the Indigenous peoples of North America. Many of the Native peoples of the northeast referred to it as "The Strawberry Moon" as June was often the month for the ripening and gathering of wild strawberries; the Haida of British Columbia called it "The Berries Ripening Moon."

The Tlingit peoples of the Pacific Northwest named it "The Birth Moon" as June is the month when many animals give birth to their young.

Canada’s Cree First Nations referred to it as "The Egg Laying Moon" or "The Hatching Moon" for the abundant eggs laid by the birds and waterfowl.

Eastern Canada’s Mi’kmaq peoples referred to it as Nipnikus, meaning “Leaves Full Blossom” or “Leaves are Budding.”

As June was typically the month of marriages in ancient Rome (a tradition that has, to some degree, continued today), the celebration of which often included the consumption of great quantities of mead (a drink made from fermented honey), the Romans referred to the full moon in June as “The Honey Moon” or “The Mead Moon.” The term “honeymoon” is tied to the Roman name for this moon.

Finally, an update to my March 14 article regarding Jupiter once again being declared the King of the Moons for its 95 known moons, taking that title away from Saturn. On May 16, the International Astronomical Union announced that Saturn has, yet again, usurped the title from Jupiter, as the Ringed Planet now has 145 confirmed moons, the first planet to have over 100 verified satellites.

Until next week, clear skies.