March 20, 2022: Spring arrives this morning at 10:33 a.m. CDT and Venus is at greatest elongation earlier in the morning. Morning planets Venus, Mars, and Saturn are in the east-southeast before sunrise.
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 6:54 a.m. CDT; Sunset, 7:03 p.m. CDT. Check local sources for sunrise and sunset times for your location.
The sun reaches the vernal equinox today at 10:33 a.m. CDT. The equinox is the origin point for two astronomical coordinate systems. One system is based on the celestial equator, the extension of Earth’s equator into the sky.
One manner to think about the sky is that it is a solid surface, a globe, very far away. All the stars, planets, moon, and sun are attached on its inside, with Earth at its center. The word “firmament” gives us this impression.
Then imagine setting up a series of lights on Earth’s equator shining upward. Their combined light makes a circle on the celestial sphere, the celestial equator.
Like longitude and latitude on Earth, the celestial sphere’s longitude is known as right ascension, measured in hours and minutes, like time. Zero hours longitude starts at the vernal equinox and it is measured eastward. One hour on the celestial sphere corresponds to 15°. Latitude, known as declination and noted north or south of the equator, is measured in degrees, like terrestrial latitude. The celestial poles have latitudes of 90°, either north or south of the celestial equator.
The celestial poles are above Earth’s poles.
In this system, conjunctions occur when planets have the same right ascension or when a planet has the same right ascension as a bright star.
The second coordinate system uses the ecliptic, the plane of the solar system. Celestial latitude and longitude are measured in degrees. Longitude is measured eastward from the vernal equinox.
Conjunctions occur when planets have the same celestial longitude. The conjunction date in this system may not occur on the same date as it does in the equatorial system. The conjunctions are closer in the ecliptic system. Articles that are written here, usually use the ecliptic coordinate system.
The ecliptic system is angled 23.5° compared to the equatorial system.
As Earth revolves around the sun, the central star seems to move along the ecliptic. Thinking of a celestial sphere, the sun could be considered to move along the solar system’s plane.
In either coordinate system, the vernal equinox occurs (today at 10:33 a.m. CDT) when the sun’s coordinates are at the origin, either Zero hours of right ascension and Zero degrees of declination or Zero degrees of celestial longitude and zero degrees of celestial latitude. Same spot on the celestial sphere, it’s the origin for both systems.
The sun is north of the celestial equator, with its light shining more directly on Earth’s northern latitudes, until the autumnal equinox on September 22. In the ecliptic system, the sun stays on the ecliptic, zero degrees celestial latitude, only its longitude changes.
Because of the definitions of sunrise and sunset, and refraction of the sun near the horizon by the atmosphere, daylight is already longer than nighttime by nine minutes. The vernal equinox point on the celestial sphere crosses the sky in 12 hours today and every day of the year.
The planets and moon are typically near the ecliptic within a band, commonly known as the zodiac.
The second event today occurs earlier in the morning at 4:25 a.m. CDT. Venus reaches its greatest separation from the sun. We see Venus farthest from our central star. Astronomically, it is known as the greatest elongation. Even though we see Venus in the eastern sky before sunrise, it rises before sunup and is west of the sun. The complete designation is greatest elongation west. This is confusing. A better name might be the morning greatest elongation.
The angle this morning is 46.6°.
If we consider the orbits of Venus and Earth to be perfect circles, the greatest elongation occurs when an imaginary line from Earth to Venus is tangent to the Venusian orbit. That tangent line is perpendicular to an imaginary line from Venus to the sun. With a little right-angle trigonometry and setting the Earth-Sun side of the triangle to a length of one unit, the Venusian distance to the sun can be estimated as a fraction of Earth’s distance. Use the value of 46.6° for the Sun-Earth-Venus angle. What is your estimate of the Venusian distance from the sun? What is the distance from Earth to Venus in earth units?
At is greatest elongation, brilliant Venus and Mars are in the east-southeast before sunrise. They are racing toward Saturn, to their lower left.
At forty-five minutes before sunrise, Venus is nearly 13° above the horizon. Dimmer Mars is 4.1° to the lower right of Venus.
Saturn, nearly 7° above the east-southeast horizon, is 8.0° to the lower left of Venus and 10.3° to the lower left of Mars.
Venus and Mars are overtaking the slower-moving Ringed Wonder. On March 28, the three planets bunch into a rare grouping that is 5.3° apart, with the moon nearby. This grouping does not occur again until 2040.
Jupiter and Mercury are in transition. After its conjunction with the sun, Jupiter is slowly climbing into the morning sky. It rises only 19 minutes before the sun crosses the horizon. The Jovian Giant is lost in the sun’s glare.
Mercury is heading toward a superior conjunction with the sun on April 2 and its best evening appearance of the year later in the month. This morning it is still west of the sun, rising a few minutes after Jupiter.
This morning the bright moon, 95% illuminated and over 20° above the southwest horizon, is 4.5° to the upper left of Spica, the brightest star in Virgo.
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