Lunar nodes affect tides on Earth
Several of my readers have written to me requesting an explanation of "lunar nodes."
Despite sounding like some strange ailment you'd get by spending too much time out in the moonlight, lunar nodes are nothing more than the two celestial reference points at which the orbit of our moon intersects the ecliptic, the plane of the Earth's orbit around the sun, and, for us observers here on Earth, the apparent pathway of the sun, moon and planets across the sky.
There are two lunar nodes: the north (ascending) node, whereby the moon, in its monthly orbit around our planet, crosses the ecliptic and moves from the southern ecliptic hemisphere into the northern ecliptic hemisphere, and the south (descending) node, at which point the moon recrosses the ecliptic and moves into the southern ecliptic hemisphere.
The lunar nodes are not fixed points along the ecliptic. Because the orbital plane of the moon changes in space (referred to as lunar precession), the nodes also precess, completing one complete revolution along the ecliptic in approximately 18.6 years (a "nodal period"), the time it takes the north (ascending) node to travel westward 360 degrees on the ecliptic, relative to the Vernal or Spring Equinox (when the sun crosses the celestial equator heading northward as viewed from Earth).
Lunar node precession has a small impact on Earth's tides throughout a given nodal period, and, with the rise in sea level as a result of global
warming, is predicted to contribute significantly to the frequency and levels of coastal flooding around the globe in the coming decades.
While the celestial mechanics of lunar nodes are more complicated and dynamic than this brief explanation, suffice it to say that lunar nodes are significant for us celestial observers, as it is only when the sun and moon are both near a lunar node that solar and lunar eclipses occur.
When the sun is close to a lunar node, and the full moon is near the node as well, a lunar eclipse occurs (as on May 26, 2021); when both the sun and the new moon are near a lunar node, a solar eclipse occurs (as on June 10, 2021).
THIS WEEK'S SKY
Mercury (magnitude -1.0, located in Gemini - the Twins) is only six degrees above the western horizon at dawn, and, as such, is extremely difficult to view.
Venus (magnitude -3.9, located in Leo - the Lion) becomes visible nine degrees above the western horizon around 9:20 p.m. ADT (9:50 p.m. NDT), setting around 10:25 p.m. ADT (10:55 p.m. NDT).
On July 21, Venus sits just 1.2 degrees north of the bright star Regulus in Leo the Lion.
Heading towards solar conjunction, Mars is currently not observable.
Saturn (magnitude +0.3, look for it in Capricornus the Sea Goat) is visible around
10:55 p.m. ADT (11:25 p.m. NDT) 10 degrees above the southeast horizon, reaching its highest point of 25 degrees in the pre-dawn, southern sky before getting lost in the dawn twilight around 5 a.m. ADT (5:30 a.m. NDT).
Look for Saturn near the full moon on July 23 and 24. Jupiter (magnitude -2.8, look for it in Aquarius - the Waterbearer) becomes visible around 11:20 p.m. ADT (11:50 p.m. NDT) eight degrees above the east-southeast horizon, reaching 31 degrees above the southern horizon by 3:40 a.m. ADT (4:10 a.m. NDT), before becoming lost in the approaching twilight by about 5:15 a.m. ADT (5:45 a.m. NDT), 27 degrees above the southwest horizon.
Bright Jupiter will sit just
four degrees north of the gibbous moon on July 25-26.
The full moon on July 23 is sometimes referred to as the Full Hay Moon, as this is the month when farmers usually harvest their first hay crop of the season. The full moon (magnitude -12.7) will rise in the southeast around 9:30 p.m. ADT (10 p.m. NDT).
Although the Perseid meteor shower is not due until mid-August, you can start watching for these fast, bright meteors anytime after July 17.
Until next week, clear skies.
EVENTS
July 21 - Venus 1.2 degrees
north of Regulus in Leo July 23 - Full Hay Moon; Saturn nearby
July 25 - Jupiter 4 degrees
north of gibbous Moon.