2023, July 6: Earth’s Aphelion, Brilliant Evening Star


July 6, 2023: Earth is at aphelion today, its farthest point from the sun. Brilliant Venus sparkles in the western sky after nightfall.

Elliptical Orbits


by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 5:22 a.m. CDT; Sunset, 8:28 p.m. CDT.  Check local sources for sunrise and sunset times for your location.  Times are calculated by the U.S. Naval Observatory’s MICA computer program.

Today, Earth is at aphelion, the point on the orbital path that is farthest from the sun.  This occurs at 3:07 p.m.  Early astronomers thought the planets revolved around the sun on perfect circular paths.  They used this model to predict the planets’ places in the sky compared to the stars.  Such models had precisions up to about 2.0°, meaning that a planet’s actual location in front of the stars could be as much as four full moon diameters away from its predicted location.

During the 17th Century, Johannes Kepler used written observations made by his astronomy mentor Tycho Brahe to plot Mars’ orbital to demonstrate that the planets revolve around the sun in elliptical orbits.  When a planet moves long its elliptical track, its distance from the sun varies.

Before the invention of the telescope, Tycho constructed large wooden protractors to measure celestial angles.  His logs were the most precise of the era.

In the solar system, astronomers use a unit measure known as an astronomical unit or A.U., in the manner we use ounces, pints, quarts, and gallons to show growing quantities for volume measurement.  The A.U. is a larger measure comprised of about 93 million miles or 150 million kilometers.  Earth’s average distance from the sun is 1.0 A.U., while Mars is 1.5 A.U. and Jupiter is 5.2 A.U.  The unit allows us to write large planetary distances with a few characters.

The nearest star beyond the sun is over 340,000 A.U.  For stars, other units are used, such as light years and parsecs.  One parsec is equal to 206,265 A.U. or 3.26 light years.  For very large distances, parsecs are abbreviated as the letters pc.  One million parsecs can be written as 1 Mpc, meaning 3.26 million light years.

Today, Earth is farthest from the sun, 1.016680468 A.U.  Have a happy aphelion day!

Summaries of Current Sky Events

Here is today’s planet forecast:

Morning Sky

Chart Caption – 2023, July 6: Saturn and the moon are in the southern sky during morning twilight.

The morning gibbous moon, 88% illuminated, is about 30° up in the south, 12.9° to the lower right of Saturn and 2.9° to the lower left of Deneb Algedi – meaning “the kid’s tail” – a star in Capricornus. A binocular is needed to see the star from the bright moonlight.

Saturn is retrograding in front of Aquarius.  This illusion occurs when our planet moves between a distant planet and the sun.  Mars, Jupiter, and Saturn show obvious retrograde patterns, although all bodies that revolve around the sun farther than Earth, display retrograde when our planet passes between them and the sun.

Tomorrow morning, the moon appears near the Ringed Wonder.

Chart Caption – 2023, July 6: Jupiter is in the eastern sky before daybreak.

At this hour, bright Jupiter is over 30° above the eastern horizon.  The planet shines as the brightest “star” in the sky this morning.  It is moving eastward in front of Aries, 11.3° to the lower left of Hamal, the constellation’s brightest star.

Notice Menkar, Cetus’ nostril, below the Jovian Giant, while the Pleiades star cluster is over 20° to Jupiter’s lower left and over 13° above Aldebaran, Taurus’ brightest star.

Evening Sky

Chart Caption – 2023, July 6: Brilliant Venus, Mars, and Regulus are in the western sky after sundown.

Mercury is climbing into the evening sky to join the planet shuffle in the western sky.  This evening it sets thirty-four minutes after the sun.

About thirty minutes later, brilliant Venus is low in the western sky.  It is visible as the sky darkens.  The planet is in its interval of greatest brightness that lasts through the 17th.

Venus is at its greatest illuminated extent, meaning that it covers the largest area of the sky.  Through a telescope the planet has its largest size with a phase that is 27% illuminated.  It appears as an evening crescent phase.  This is a semi-technical explanation, stating that the planet is very bright in the sky.  NASA’s Jet Propulsion Laboratory, also known as JPL, predicts that Venus is brightest on the 12th.  While the planet’s visual intensity is increasing, the human eye cannot see the tiny changes, but the change is easy to see across several weeks.

Venus begins to retrograde on the 22nd and is slowing its eastward pace.  It approached to 3.6° of Mars on June 30th, but it did not pass the Red Planet. Tonight, their separation is 4.0°. The Evening Star is angling below the plane of the solar system, and it does not reach Regulus, 5.9° to the upper left of the brilliant planet this evening.

Mars continues its eastward march, passing Regulus in four nights. Look for this triplet, Venus, Mars, and Regulus, through a binocular.

Regulus is part of a westward facing Lion that we see in silhouette.  At this season, the constellation is tipped toward the western horizon.  A backwards question mark, outlines the head and a triangle dots the haunches and tail, identified as Denebola.

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