Bright Jupiter, Saturn, and Mars shine in May’s morning sky.
Click here for our semi-technical description of the planets during May 2020.
A gallery of May’s morning planets
At the beginning of the month Jupiter (m = −2.4) rises at 1:25 a.m. CDT, followed by Saturn (m = 0.6) about 15 minutes later. Mars (m = 0.4) rises nearly an hour after Saturn. One hour before sunrise, these planets span nearly 25° in the southeast. Bright Jupiter, nearly 25° up in the south-southeast is 4.9° to the right of Saturn. Farther east, Mars is 18° up in the southeast. Among the stars, Mars is 0.9° above Gamma Capricorni (γ Cap, m = 3.6) and 1.5° to the upper right of Delta Capricorni (δ Cap, m =2.8). Use a binocular to see Jupiter and Mars in their respective starfield.
In the notes in this article, the “m” numbers are measures of the planets’ brightness. The lower the number, the brighter the celestial object. The sun has the lowest value (−26.5) on this scale. Afterall it is so bright it creates daytime on our planet and shines on the moon and other planets in its system. Jupiter’s brightness is scaled at −2.6, while Saturn and Mars are slightly positive, but Mars is brighter. The planets’ brightness changes as their distances from Earth vary. Later this year, Mars is as bright as Jupiter when Earth passes between the Red Planet and the sun, an event known as opposition. So, like an excellent golf score, the lower the number the brighter the star.
In the sky we measure the sizes of objects by how large they appear to our eyes. This is measured in degrees as those on a protractor. The moon’s apparent diameter is about 0.5°, not as big as we might think.
When we observe the sky, the objects seem to be different sizes because of their actual sizes and their distances. The moon’s apparent size seems large, about 0.5° as measured in angular units. The sun is farther away than the moon, but it appears the same size. Because of this, the moon can cover the sun to create a solar eclipse. The tip of your pinky finger on your outstretched arm easily covers a full moon. Try it the next time you see a brilliant full phase.
The space that your outstretched fist covers is about 10°. The Jupiter – Saturn gap is nearly 5°. They are about half a fist apart. The Jupiter – Mars gap is about 25°, or 2.5 outstretched fists apart.
Jupiter and Saturn seem to reverse their direction of motion compared to the stars. In this retrograde motion, the planets seem to move backwards or westward compared to the starry background. Normally the planets move eastward. If you’ve been watching Mars, it has been moving eastward and farther away from Jupiter and Saturn. This illusion occurs as the planets revolve around the sun and the faster moving Earth catches and passes the outer planets. Saturn begins to retrograde May 10, Jupiter, May 14. The chart above shows the complete motions of Jupiter and Saturn during their appearances.
Note the motion of Saturn compared to Sigma Capricorni (σ Cap) and Jupiter to 56 Sagittarii (56 Sgr). The chart above shows the location of the planets with these two stars.
Jupiter and Saturn are headed toward their once-every-generation Great Conjunction. Read this article for more details about this conjunction. Both planets retrograde before Jupiter passes Saturn later this year.
To early astronomers, retrograde motion was a major cosmological problem. How did this occur? The challenge focused on Earth’s location. The issue of a sun-centered universe or earth-centered universe was not settled until after the invention of the telescope when evidence of Earth’s revolution around the sun was first observed.
With a binocular watch the motion of the planetary pair compared to two dimmer stars nearby. Our detailed monthly notes outline locations of the planets compared to the starry background.
Near the middle of the month, the moon moves past the planets: Here are the daily notes:
- May 12: One hour before sunrise the gibbous moon (19.3 days past the New phase, 72% illuminated) is 3.1° to the lower right of Jupiter and 6.1° to the lower right of Saturn. Farther east, Mars is over 20° in altitude in the southeast.
- May 13: One hour before sunrise, the gibbous moon (20.3d, 62%), over 20° up in the south-southeast, is in central Capricornus between Jupiter and Mars. Jupiter is over 25° up in the south-southeast, 4.7° to the right of Saturn that is nearly 9° to the upper right of the moon. Mars is over 20° up in the southeast.
- May 14: The moon (21.3d, 52%) is 2.4° below Delta Capricornus (δ Cap) and about 9° to the lower right of Mars.
- May 15: Jupiter (m = −2.5) rises about 1:30 a.m. CDT, followed by Saturn 15 minutes later. Mars rises about 2:15 a.m. CDT. One hour before sunrise, the moon (22.3d, 42%) is 4.5° to the lower left of Mars, nearly 21° up in the southeast.
On March 26, as Mars, moving eastward compared to the starry background, appears in the middle of a triangle of three dimmer stars, Lambda Aquarii, Sigma Aquarii, and Tau Aquarii. Use a binocular to spot Mars in the middle of these stars.
The month ends with Jupiter and Saturn retrograding and Mars in the southeast, over 45° from Jupiter. Both giant planets – Jupiter and Saturn – are rising before midnight. Mars rises about 12:45 a.m. CDT. Notice that the bright star Fomalhaut is near the horizon to the lower right of Mars.
Happy observing!