Are you looking for it, too? It's that frustrating time of year again. As the Northern Hemisphere rolls away from winter we eagerly await signs of spring. For many, spring comes with returning robins bobbing along the ground in a spirited hunt for lunch. For others it's spring flowers putting in a first appearance. But the most reliable sign of spring is not of the Earth, but of the sky. It is the stars of Leo.
Leo the lion is a prominent spring constellation. When I see Leo riding high in the southern sky before midnight (figure 1), I know that Earth has not stalled in its yearly orbit and we are not stuck in perpetual winter.
At night, we face away from the Sun. Our nighttime view changes through the year. As Earth orbits around the Sun our view sweeps through the yearly cycle of seasonal constellations. The solar system is tilted compared to the plane of the galaxy (figure 2), and this also affects what we see as we look out into the stars. During spring nights we face away from the rich star fields of our galaxy. The Milky Way hugs the western horizon and our view is up and out of the galaxy (figure 3). This gives us a clear view into the deeps of intergalactic space.
In spring, the rest of the universe is on display. There is little interference from the Milky Way's intervening clouds of gas, dust and stars. Millions of external galaxies glow faintly in the blackness of space, beckoning telescopes to reveal their splendor. Astronomers answer the call of the galaxies each spring with eager enthusiasm. Leo and its neighboring constellations are rich territory for observers wanting to soak up photons from hundreds of millions of light years away. Such intergalactic communion is an experience like none other.
We all have our favorite signs of spring. This year spring arrived officially on March 21 at seven minutes after midnight Universal Time. For sky gazers, spring is synonymous with the arrival of Leo and a sky blooming with galaxies. The celestial lion is the surest signal that terrestrial blossoms will soon follow.