Tonight – June 17, 2018 – try to imagine the ecliptic (the green line on our sky charts), which represents the sun’s annual pathway in front of the background stars. Being able to visualize the ecliptic from your favorite spot to observe the heavens is helpful, because the moon always moves approximately along this path, and so do the planets. Once you get a sense of it, you’ll know to look for certain bright stars and planets along this nighttime pathway. And indeed, on these June and July 2018 evenings, three bright stars and three planets – and the moon – can help you visualize the ecliptic crossing your sky.
So, first of all, what is the ecliptic? It’s defined by the plane of Earth’s orbit around the sun. But astronomers – who frequently need to think about multiple vantage points simultaneously – also speak of the ecliptic as the yearly path of the sun in front of the constellations of the zodiac.
The ecliptic isn’t the same thing as the celestial equator, which is another imaginary great circle, above Earth’s actual equator. It isn’t, because Earth is tilted on its axis with respect to our orbit around the sun. The video below is a visual explanation of why the plane of the ecliptic is tilted with respect to the celestial sphere, the imaginary sphere of stars surrounding Earth. The animation was created to teach college and high school astronomy, and there’s no sound.
So don’t expect an explanation in the video. Just look, and think about the various planes involved.
Was that helpful in giving you a sense of the ecliptic?
Now – look below – and let’s think about some real objects you’ll find in the real sky in June 2018. The first chart below shows the western half of sky at nightfall. The second chart below extends the line of the ecliptic eastward, showing the eastern half of sky for around mid-evening. The third chart below depicts the moon’s position in front of the constellation Leo and Leo’s brightest star, Regulus, for June 17, 18 and 19. On these dates, at early evening, the moon’s lit side is pointing toward Venus, which sits low in the west, and the moon’s dark side is pointing toward Jupiter’s place higher up in the southern sky.
By the way, Mars rises into the southeast sky shortly after Venus sets in your west to northwest sky. Click here for a recommended almanac, giving you Venus’ setting time and Mars’ rising time into your sky.
So there you have it, the ecliptic – marked by three bright planets (Mars, Jupiter and Saturn) and three bright stars (Regulus, Spica and Antares) – on June 2018 evenings.
If you watch over the next several nights, you’ll enjoy seeing the moon move – more or less – along the ecliptic. The moon doesn’t orbit Earth in exactly the same plane that Earth orbits the sun, but nearly so. So its monthly path across our sky is nearly the same as the sun’s annual path. The moon moves eastward in orbit, so, in the coming evenings, it’ll be moving eastward (toward the sunrise direction), passing each of these stars and planets we’ve just talked about.
On June 21, the moon will be near Spica, the brightest star in the constellation Virgo
On June 23, the moon will sweep close to Jupiter, the second-brightest planet (after Venus)
On June 25, the moon will swing north of Antares, the brightest star in the constellation Scorpius
On June 27, the moon will pair up closely with Saturn, which is also at opposition
On June 30, the moon will swing north of Mars, soon to replace Jupiter as the second-brightest planet
See the sky charts below:
Have fun watching … wishing you clear skies!
Bottom line: Use your mind’s eye to imagine the ecliptic, or sun’s path, crossing your night sky.