Tonight for July 21, 2015Courtesy U.S. Naval Observatory
Tonight’s moon – July 21, 2015 – is heading on our sky’s dome toward Spica, the constellation Virgo’s brightest star. The moon crosses the ecliptic – the plane of Earth’s orbit around the sun – on July 21, too. How often does that happen, and what does it mean for our view of the moon in front of the stars? Keep reading …
The plane of the moon’s orbit around Earth is inclined at 5o to the plane of the Earth’s orbit around the sun. Still, the moon crosses the Earth’s orbital plane twice (sometimes three times) a month.
This month, the moon crosses the ecliptic (going from south to north) on July 21 at 19:32 Universal Time (2:32 p.m. CDT). For us in North America, this crossing of the moon over the ecliptic (Earth’s orbital plane) occurs during daylight hours.
On July 20, the moon was south of the ecliptic. The sky chart below helps you to visualize where the moon crosses the ecliptic and also where the dazzling planets Venus and Jupiter are found at dusk. Spica is to the east – left – of the moon, outside the sky chart.
Nodal passages of moon: 2001 to 2100
The green line depicts the ecliptic – Earth’s orbital plane. The moon crosses the ecliptic, going from south to north, on July 21, 2015, at 19:32 Universal Time (2:32 p.m. CDT).
Wait another night – until July 22 – and the moon will be north of the ecliptic, and closer to the bright star Spica. That’s because the moon always moves eastward relative to the backdrop stars of the Zodiac, as it travels eastward in orbit around Earth.
Throughout 2015, the moon crosses the ecliptic in front of the constellation Virgo. If you could see stars during the day, you’d see the sun in front of Virgo every September equinox. This year, the moon crosses the ecliptic – going from south to north – near the September equinox point in Virgo every month. See the sky chart below.
When the moon crosses the ecliptic, going from south to north, the moon is said to be at its ascending node.
View larger. The September equinox point in the constellation Virgo marks the intersection of the ecliptic with the celestial equator. In the year 2015, the moon’s ascending node closely coincides with the September equinox point.
In a year when the moon’s ascending node crosses the ecliptic at or near the September equinox point, we have what is known as a minor standstill year. That’s the case for 2015.
In a minor standstill year, the range of the moon’s monthly travels north and south of the celestial equator shrinks to a minimum, as the moon goes from about 18.5o south to 18.5o north of the celestial equator every month. The celestial equator, by the way, is a projection of the Earth’s equator upon the celestial sphere.
Some 9.3 years from now, in the year 2025, it’ll be the moon’s descending node (north to south) – instead of its ascending node (south to north) – that’ll closely align with the September equinox point. That’ll bring about a major standstill year, whereby the moon swings a maximum of about 28.5o south to 28.5o north of the celestial equator.
Monthly lunar standstills: 2001 to 2100
At present, the moon goes north of the ecliptic and the star Spica during its monthly passages in front of Virgo. Looking ahead, a series of 20 lunar occultations of Spica will start on June 16, 2024, and end on November 17, 2025.
Bottom line: Watch the moon as it lights up dusk and evening on July 21, as it moves toward the Virgo star Spica.
Thanks to: http://earthsky.org