2021, April 15: Jupiter, Moons Dance with Star

2021, April 15: Jupiter and Saturn are in the southeast before sunrise. The planets are in front of Capricornus.
2021, April 15: Jupiter and Saturn are in the southeast before sunrise. The planets are in front of Capricornus.

April 15, 2021: Morning planets Jupiter and Saturn are in the southeast before sunrise.  Jupiter is the brightest “star” in the region.  Through a telescope, Jupiter and its moon dance with a star.

by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:10 a.m. CDT; Sunset, 7:32 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

Two bright planets are in the southeast before sunrise.  Saturn is nearly 17° above the horizon.  In the starfield it is 1.7° to the upper right of the star Theta Capricorni (θ Cap on the chart). 

Jupiter is 13.5° to the lower left of Saturn.  The Jovian Giant is the brightest “star” in the region.

Both planets are gently moving eastward compared to the starry background.  This morning Jupiter is 2.8° to the upper left of Deneb Algiedi (δ Cap) and 0.2° to the upper right of Mu Capricorni (μ Cap).

2021, April 15: A telescopic view of Jupiter, its moons, and the star Mu Capricorni (μ Cap).
2021, April 15: A telescopic view of Jupiter, its moons, and the star Mu Capricorni (μ Cap).

This morning and during the next two mornings, Jupiter, its four largest moons, and Mu Capricorni fit into a low-power eyepiece of a spotting scope or telescope.

The star seems to intermingle with the moons.  It is over a million times farther away than Jupiter, but is along the same line of sight. 

Like the planets, Jupiter’s largest moons line up along the same plane.  Through a telescope, they appear along a line on either side of the planet.

From North America and South America through a low-power telescope this morning, the satellite Callisto is nearly midway from Jupiter to the star. Ganymede and Europa are between Callisto and the planet, but closer to Jupiter.  The moon Io is on the opposite side of the planet.  Observers from other parts of the globe will see the moons in a slightly different orientation.

The chart above shows the view with no inversion as some telescopes present the sky.  Some telescopes flip the view upside down and backwards left to right.  Some show no inversion, but flip the view from left to right. A spotting scope that is use for birdwatching or other terrestrial viewing shows no change in the image.

Detailed Note: April 15: One hour before sunrise, Saturn is nearly 17° above the southeast horizon and 1.7° to the upper right of θ Cap.  Bright Jupiter is 13.5° of ecliptic longitude east of Saturn and to the Ring Wonder’s lower left in the sky.  The Jovian Giant is nearly 12° above the east-southeast horizon.  In the starfield, Jupiter is 2.8° to the upper left of Deneb Algiedi and 0.2° to the upper right of μ Cap.  In a low power telescopic eyepiece, notice that the star nearly lines up with the morning string of Jupiter’s moons. Callisto is nearly midway from the planet to the star.  Venus was at superior conjunction three weeks ago, but it has not yet made its first naked-eye appearance.  This evening the planet’s elongation is 5°, nearing its first evening appearance without optical aid.  One hour after sunset, the moon (4.0d, 13%) is less than one-third of the way up in the west.  It is a nice moon for photographing earthshine.  The lunar slice is above a line from Aldebaran (α Tau, m = 0.8) that extends through Epsilon Tauri (ε Tau, m = 3.5).  The crescent is 5.1° to the upper right of Aldebaran and 2.0° to the upper right of ε Tau. Mars is to the upper left of the moon above the horns of Taurus.  The Red Planet is 4.7° to the upper left of Elnath and 3.6° to the upper right of ζ Tau.

Read more about the planets during April 2021.

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