Spring Arc to Arcturus: Navigating the Spring Sky

Every season has its signature sky navigation trick, and for spring, it is the Arc to Arcturus. This simple technique uses the Big Dipper -- the most recognizable pattern in the northern sky -- to guide you to Arcturus, the brightest star in the Northern Hemisphere. From there, you can unlock the rest of the spring sky.

The Technique

The Big Dipper (part of Ursa Major) rides high overhead during spring evenings, with its handle curving gracefully away from the bowl. To find Arcturus:

1. Locate the Big Dipper. In spring, it is nearly overhead for mid-northern latitude observers, with the bowl to the upper left and the handle to the lower right.
2. Follow the arc of the handle. The three stars of the handle form a gentle curve. Extend this curve beyond the handle in the same sweeping direction.
3. Continue about one handle-length. The arc leads to a brilliant golden-orange star -- Arcturus.

The phrase to remember is simple: "Arc to Arcturus." And it does not stop there.

Spike to Spica

Continue the arc beyond Arcturus, and the next bright star you encounter is Spica, a blue-white star in Virgo. The rhyme extends: "Arc to Arcturus, spike to Spica." This two-step navigation gives you two of the brightest stars in the spring sky and orients you to two important constellations.

Arcturus: The Star

Arcturus (Alpha Bootis) is the brightest star in the constellation Bootes and the fourth-brightest star in the entire night sky at magnitude -0.05. Its warm orange color is visible to the naked eye and becomes vivid through binoculars. The color comes from its classification as a K-type giant star -- it has exhausted the hydrogen fuel in its core and expanded to about 25 times the Sun's diameter.

Arcturus lies about 37 light-years from Earth, making it a relatively close neighbor. It moves through space at an unusually high velocity relative to nearby stars, indicating it may have originated in a different part of the galaxy and is merely passing through our neighborhood.

Spica: The Star

Spica (Alpha Virginis) shines at magnitude +0.97 with a distinctive blue-white color. It is actually a close binary system -- two massive stars orbiting each other so tightly that they cannot be separated even through a telescope. Spica sits along the ecliptic, so planets and the Moon regularly pass near it, creating attractive conjunctions.

From Arcturus to the Spring Constellations

Once you have identified Arcturus and Spica, the rest of the spring sky falls into place:

Bootes the Herdsman

Arcturus sits at the base of a kite-shaped pattern. The kite extends northward from Arcturus, easy to trace on clear nights. Bootes contains several fine double stars for telescope users.

Virgo the Maiden

Spica anchors a large but faint constellation that stretches toward Leo. Virgo is home to the Virgo Galaxy Cluster, making it prime territory for telescope-equipped galaxy hunters during spring.

Corona Borealis (The Northern Crown)

A small but elegant semicircle of stars east of Bootes. Its seven stars form a nearly perfect arc, making it one of the most distinctive small constellations.

Leo the Lion

Looking westward from Arcturus, Leo the Lion dominates the southwestern sky during spring evenings. Its Sickle asterism (a backward question mark) is easy to find, with bright Regulus at its base.

The Spring Diamond

Some observers recognize a Spring Diamond formed by Arcturus, Spica, and two other bright stars -- Denebola (the tail of Leo) and Cor Caroli (in Canes Venatici). This diamond-shaped asterism is less famous than the Winter Hexagon or Summer Triangle but provides useful orientation for the central spring sky.

Deep-Sky Objects Along the Arc

The region between the Big Dipper, Arcturus, and Spica is rich in galaxies because it lies away from the dust of the Milky Way plane:

• M51 (Whirlpool Galaxy): Near the end of the Big Dipper's handle in Canes Venatici.
• M3: A bright globular cluster between Arcturus and the Big Dipper. Beautiful in binoculars and telescopes.
• M104 (Sombrero Galaxy): South of Spica in Virgo, visible as an elongated smudge through telescopes.
• The Virgo Cluster: Dozens of Messier galaxies concentrated between Virgo and Coma Berenices.

Planets Along the Ecliptic

Since Spica lies near the ecliptic, planets frequently pass through this region. Check planet positions tonight -- you may find Jupiter, Saturn, or Mars near Spica or in Virgo during certain years. The contrast between a planet's steady light and Spica's blue-white twinkle makes it easy to tell them apart.

Common Questions

Does the arc work year-round?

Yes. As long as the Big Dipper is above the horizon (which it is for most of the year from mid-northern latitudes), the arc technique works. However, spring is when Arcturus is highest and most prominent in the evening sky.

What if I am in the Southern Hemisphere?

Arcturus is visible from much of the Southern Hemisphere but sits lower in the northern sky. The Big Dipper may be partially or fully below the horizon from far-southern latitudes.

Try the Arc Tonight

Open StarGlobe to locate the Big Dipper and trace the arc to Arcturus from your position. This one navigation trick connects you to the heart of the spring sky and leads to galaxies, bright stars, and constellations that are unique to this transitional, rewarding season. For navigation aids in other seasons, see the Winter Hexagon, Summer Triangle, and Great Square of Pegasus.