2017, February 12: The Winter Sky

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The Winter Stars gleam brightly from the southern sky this evening.  Orion, with its bright stars, Betelgeuse and Rigel, slowly march westward.  Within the constellation is the Orion Nebula that appears as a fuzzy cloud through a small telescope or binoculars.  Aldebaran appears in front of the Hyades star cluster, that appears in a check mark shape or a letter “V” if you include the bright star.  The Pleiades star cluster is to the upper right of the Aldebaran.  You can count six or seven stars.  Through binoculars you may see a few dozen.

The clear winter sky tonight provides for excellent sky watching.

2017, February 12: Venus and Mars

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Venus and Mars shine from the western sky this evening.  Bright Venus is slowly moving away from Venus.  Tonight they are 6.5 degrees apart. Tonight Mars sets about 20 minutes after Venus.  By month’s end the gap grows to about 40 minutes.

For more information about current sky events, see these articles:

2017, February 10: Lunar Eclipse Photo

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Penumbral Lunar Eclipse, February 10, 2017. Photo by Tim S.

This photo of the February 10, 2017 penumbral lunar eclipse shows darkening on the upper edge of the moon.  Skies clear in the region after the maximum part of the eclipse.

For more information about current sky events, see these articles:

2017, February 3: Venus and Mars

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Venus and Mars shine in the southwestern sky as seen from Madison County, Ohio Friday evening.  The planets appear at their closest (5 degrees) until both appear in the morning sky in October when they have a spectacular conjunction.

The planets separate rapidly during the next few weeks as Venus moves toward the sun for inferior conjunction in late March.

For more information about the Venus-Mars encounter and other planet observations, see these articles:

2017: Venus as a Morning Star

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Venus last appeared in the morning sky in 2015 and 2016, when it appeared with Mars and Jupiter.

Introduction

Brilliant Venus zips into the morning sky during April  2017 and dominates the morning sky until year’s end.   During this morning appearance, Venus makes close appearances with the star Regulus and the planets Jupiter and Mars.

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This chart shows the rising time of bright planets, the moon, and stars near the planets’ orbital plane (ecliptic) compared to sunrise as calculated from U.S. Naval Observatory data for Chicago, Illinois in the Central Time Zone.  Additionally, the times when Jupiter sets and Saturn sets are charted compared to sunrise.  On April 7, Jupiter is at opposition and it sets in the west at sunrise.  The time differences are also displayed for Civil Twilight, Nautical Twilight and Astronomical Twilight.  At Astronomical Twilight, the sky is as dark as it gets naturally.

The rising time of Venus is represented by the green line on the chart.  It enters the chart in mid-March, reaching its maximum rising time difference during the summer, and leaves the sky in early 2018.  Notice that during the summer months of this appearance of Venus, the brilliant planet rises well before the beginning of twilight.  It stands low in the eastern sky as the sky brightens.

As Venus appears earlier in the morning sky, Jupiter shines brightly in the western sky, until about May 20 when Jupiter sets as Venus rises. (Notice on the chart, Jupiter sets line crosses the Venus rises at May 20.)  After this date Jupiter sets before Venus rises.  Similarly, Saturn, while not as bright as Jupiter or Venus, reaches opposition on June 15, setting in the west as Venus rises in the eastern sky.  Venus appears in the eastern morning sky and Saturn appears in the western sky until about July 25 when Saturn sets as Venus rises.  After this date Saturn sets before Venus rises.

Later in the year, Venus appears near Regulus.  This occurs near the date when the rising lines of Regulus and Venus intersect.  The same occurs for Mars, Spica, and Jupiter.  As Venus moves back into bright sunlight later in the year, it appears near Mercury, Antares and Saturn, although they appear together during bright twilight and out of view for most observers.

Venus has a close conjunction with Mars on October 5, followed by a very close (Epoch) conjunction with Jupiter on November 13.

The moon passes Venus each month, as our nearest celestial neighbor moves through its celestial path.  Two dates (May 22 and July 20) are especially noteworthy when Venus and the moon appear about 3.5 degrees apart.

Inferior Conjunction

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Venus moves between Earth and Sun on March 25, 2017; this is known as inferior conjunction.  Since Venus has a shorter orbital path and faster speed, it quickly moves into the morning sky.  The red line on the chart shows the division between morning and evening.  The line pointing from the earth to the sun indicates noon.  So at inferior conjunction, Venus rises with the sun, appears in the south at noon, and sets in the west at sunset.

Venus does not appear in the sky at midnight at mid-northern latitudes.  That occurs when a planet is opposite the sun in the sky as seen from Earth.  On the chart notice that the midnight line does not point toward Venus.

As Venus reaches this inferior conjunction, it passes above the sun.  Because it is north, above the sun, it rises earlier than the sun.  On conjunction morning it rises about 40 minutes before the sun.  On the rising chart above, it first appears on the chart on March 14, 11 days before it reaches conjunction!

Venus was last at inferior conjunction on August 25, 2015, 589 days between inferior conjunctions.

Greatest Brightness

The planet rapidly moves into the morning sky, rising earlier each morning.  It is very close to our planet and sparkles in the morning sky.  The brightness is from the proximity of the planet to Earth, its highly reflective clouds and the phase of the planet.  (Yes, Venus shows phases when viewed through a telescope.)  At this time Venus is about 170 times the moon’s distance, relatively close compared to other planets.

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From April 15 through May 13, Venus appears brightest in our skies, with the mid-point on May 1, 2017.  This is shown with the GB (greatest brightness) designation on the rising chart above.

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Venus continues to rise earlier each morning. On the morning of May 22, the crescent moon appears about 3.5 degrees from Venus.

Greatest Elongation

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Venus reaches its greatest angular separation (46 degrees) from the sun on June 3.  This is shown by the GE symbol (greatest elongation) on the rising chart above.  It rises about 2 hours before sun near the beginning of twilight.

Venus Dazzles Morning Sky

 Venus continues rising earlier as summer begins.

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On July 14, Venus moves past Aldebaran.  The closest approach is about 4 degrees.

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The Binocular View

More striking is the star cluster near Venus and Aldebaran: Hyades.  The Hyades cluster is about 2.5 times farther away than ruddy Aldebaran.  Through binoculars, Venus, Aldebaran and the jewel-like stars of the cluster sparkle against the black velvet of the  predawn sky. Several dozen stars can be seen.

To the unaided eye, the Hyades resemble a check mark or a letter “V” if Aldebaran is included.

Clusters, like the Hyades, are used to refine distance measuring techniques as well descriptions of the lives of stars.  These clusters are thought to form at approximately the same time.  Stars that burn their nuclear fuels faster convert into other stellar forms sooner, such as red giants and red super giants.  From these stellar models, the estimate of the sun’s total lifespan is about 10 billion years.

Over time these clusters break apart; the gravitational forces between the stars are not strong enough to keep the cluster together.  The stars go their own way in their orbital path around the galaxy.

Our sun was likely formed in such a cluster and is now a lone star since it has gone into its own orbit around the Milky Way galaxy.

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On the morning of July 20, the crescent moon again appears with Venus.  The pair is separated by about 3.5 degrees.

In early August Venus rises about 3 hours before sunrise and begins to rise later each morning as displayed on the rising chart.  For the rest of the year, it loses about 30 minutes each month.

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In late summer and early Autumn look for Venus and Sirius at the same time.  Both are about the same height (altitude) above the eastern horizon.  Venus stands in the east-northeast and Sirius appears in the southeast.  Only the sun and moon shine brighter than Venus and Sirius is the brightest star in the night sky. See this link to view the last time Venus and Sirius appeared together in the morning sky.

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Venus continues its rapid eastward movement as compared to the stars and descent toward the sun’s glow, passing about a half degree from Regulus on September 20.  This pair rises about 2 hours before sunrise.

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Over a month later, Venus passes Spica.  The gap is nearly 4 degrees.

Mars Conjunction

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The first planetary conjunction of this appearance is with Mars.  On the morning of October 5, Venus passes 0.2 degrees from the Red Planet.  The planets are close on a few mornings before and after the conjunction.

Venus-Jupiter Epoch Conjunction

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Another Epoch (close) Conjunction of Venus and Jupiter occurs before sunrise on November 13.  The distance is about .2 degrees.  This conjunction is visible during twilight as the pair rises about 70 minutes before the sun.

The next conjunction between the pair is November 24, 2019 with the next epoch conjunction on April 30, 2022.

Superior Conjunction

Venus continues its rapid descent into bright sunlight.  Conjunctions occur with Mercury, Antares and Saturn, but they occur in bright twilight, out of the view of most observers.

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On January 9, 2018 passes behind the sun at its superior conjunction and reappears in the evening sky.

Lunar Conjunctions

The moon appears with Venus on the following dates:

April 23: 8 degrees
May 22: 3.5 degrees (See description in text)
June 20: 7 degrees
July 20: 3.5 degrees (See description in text)
August 19: 4.5 degrees
September 17: 6 degrees
October 18: 5.5 degrees

Venus provides a dazzling view of planetary, stellar and conjunctions during its morning appearance in 2017.

2017, February 1: Venus, Mars & the Moon

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Brilliant Venus shines through thin clouds this evening.  Dimmer Mars is 5.5 degrees to the upper left of Venus and the Moon is 11.5 degrees above Mars.  The planets are closest on February 3.  Venus does not pass Mars at this appearance.

For more information about the Venus-Mars encounters and other planet observations, see these articles:

2017: Mercury in the Evening Sky

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This chart shows the setting times of Mercury, other planets, and stars in the western sky compared to sun during 2017.  The vertical axis shows hours after sunset.  The horizontal axis shows the dates every seven days.

Mercury makes three appearances in the evening sky during 2017.  The brown curves on the chart above represent Mercury’s setting times compared to the times of sunset.  The three appearances peak around their greatest elongation dates:  April 1, July 29, and November 23.

The Geometry

The greatest elongation is the planet’s greatest angular separation from the sun.  As Mercury speeds around the sun it rapidly moves from the sun’s glare into the evening or morning sky and then back into the sun’s brilliance.

The planet revolves around the sun every 88 days.  Because our planet is revolving, yet at a slower speed and on a longer path, Mercury catches up and passes us every 116 days.

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Mercury never appears no more than about 27 degrees from the sun and never appears in the sky at midnight from the mid-northern latitudes.  So it is in the sky mainly during the daytime and sometimes during twilight.  The chart above shows Mercury on April 1, 2017, when it is at its greatest elongation or maximum separation (19 degrees) from the sun.  This chart shows the sun and Mercury at noon, if the sky were dark.  The red line represents Mercury’s invisible orbital path.

Hold a ruler (12 inches) at arms length.  If the sun were at one end, Mercury would appear near the other end when it is at its greatest separation from the sun.

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Mercury passes behind the sun on March 6, 2017.  It is invisible because it is hidden in the sun’s glare.  The planet then moves into the evening sky, east of the sun, until it reaches its greatest separation from the sun on April 1.  It then rapidly moves into the sun’s glare and passes between Earth and sun  (inferior conjunction).  Mercury then moves into the morning sky, west of the sun.  After reaching its greatest elongation it moves back into the sun’s brilliance reaching superior conjunction.  The chart above shows the configurations if the earth were stationary.

The April 1 Elongation

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Our view of the planet is further complicated by our seasonal view of the plane of the solar system, known at the ecliptic.  The chart above shows the plane and Mercury’s orbit at the planet’s greatest elongation (19 degrees), just after sunset.  The narrow elongation would be particularly challenging to see, but the angle the ecliptic makes with the western horizon is very large.  During spring evenings, the ecliptic makes its highest angle with the horizon.  Venus and Mercury stand very high in the western sky during these times.  In the morning, the ecliptic makes its sharpest angle on autumn mornings.

On the evening of April 1, Mercury sets over 100 minutes after sunset.  Astronomical twilight ends 96 minutes after sunset, and nautical twilight, 61 minutes.  At astronomical twilight, see the chart at the top of this article, the sky is as dark as it will ever get naturally.  At nautical twilight, the horizon is visible, so that mariners can take measurements of star’s heights (altitudes) above the horizon.

So at this appearance, Mercury can be seen low in the western sky.  First locate the planet with binoculars, then try with optical help.  Look for Mercury a week before and after April 1.

The July 29 Elongation

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At the next greatest elongation on July 29, the angular separation is 27 degrees, but the angle of the ecliptic is very unfavorable.  Mercury sets about 60 minutes after sunset, but nautical twilight occurs 70 minutes after sunset.  Astronomical twilight occurs nearly 115 minutes (almost two hours) after sunset.  Mercury is less than 15 degrees at sunset.  This is likely a binocular or small telescope only apparition, from the unfavorable position of the ecliptic.

The November 23 Elongation

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Similarly, the November 23rd apparition has unfavorable observing conditions.  At sunset, the planet is less than 10 degrees above the horizon and sets nearly 70 minutes after sunset.  Saturn is 4 degrees above Mercury and sets nearly a 30 minutes after the speedy planet.  Nautical twilight occurs at nearly the same time as Mercury sets and astronomical twilight is nearly 35 minutes later.  Like the summer appearance, Mercury can be seen with optical aid.

Mercury appears in the western evening sky three times during the year.  Because of favorable seasonal circumstances, the April 1, 2017, view is the best of the year.