While most people probably know that the moon influences the way the tides rise and fall each day, very few people understand exactly how and why it happens.
It all has to do with tidal forces between two celestial objects: the Earth and the moon. When two celestial objects are in close proximity, there is a mutual gravitational attraction relative to their respective masses and the distance between them. This attraction is referred to as tidal force.
The moon exerts different gravitational tidal forces at different locations on Earth. The side of the Earth that's facing the moon experiences a stronger tidal force than the centre of the planet, while the side of the Earth that's facing away from the moon experiences less tidal force than the centre of the planet. This explains why the side of Earth facing away from the moon also experiences a high tide at the same time the side facing the moon experiences a high tide.
These contending tidal forces attempt to stretch the Earth along a line connecting the centre of the Earth and the moon. However, because the Earth is fairly rigid, it cannot deform very much in response to the tidal forces of the moon, but can only flex a small amount, resulting in the Earth becoming slightly non-spherical and somewhat flattened at the poles. The Earth's equatorial diametre is 12,756 kilometres and 12,720 kms from pole-to-pole, while its equatorial circumference is 40,075 kms, and 40,008 kms pole-to-pole.
The Earth's oceans, however, are not rigid. The oceans can, and do, respond to the tidal forces of the moon much more readily. As the Earth rotates, any point on the Earth's surface directly below, including the Earth's oceans, is subject to the moon's gravitational pull. As a consequence, the ocean water at that point goes from shallow to deep, and back again, as the moon orbits the Earth, resulting in the twice-daily low and high tides experienced at most, but not all, locations.
On any given beach, the actual timing and height of the tides depend on the shape of the coastline and the particular beach, the slope of the seabed running up to the beach, and the prevailing winds and coastal currents.
The sun also exerts tidal forces on the Earth's oceans, although only about 50 per cent as great as the lunar forces. When the sun, Earth, and moon are aligned (as they are at full and new phases), the sun and moon's tidal forces reinforce one another, resulting in lower and higher tides than normal. The high tides at this time are sometimes referred to as "spring tides" with no reference to the season, but, rather, that the water level "springs up" to a greater height than normal.
This is even more pronounced with supermoons, when the full or new moon is at its closest approach to Earth (perigee). During the moon's first quarter and last quarter phases, when the moon and the sun are at right angles to one another relative to the Earth, as viewed from above the poles, the tidal forces of the sun and moon partially cancel each other out, resulting in smaller tidal shifts, referred to as "neap tides."
This week's sky
Neither Mercury nor Venus are observable this coming week.
Mars (magnitude +1.3, seen in Taurus - the Bull) becomes visible around 8:20 p.m., 49 degrees above the western horizon, before dropping down and setting about 1:30 a.m.
Saturn (magnitude +0.8, seen in Capricornus - the Sea Goat) and Jupiter (magnitude -2.1, located in Capricornus) are both visible just above the pre-dawn southeast horizon between 5:10-5:30 a.m., although you will need a clear and unobstructed view of the horizon to see either planet, as they won't rise very high above the horizon (Saturn will be at nine degrees, and Jupiter at 10 degrees) before they are both washed out as dawn breaks around 6:30 a.m.
If you're looking for a really challenging night sky object to find, have a look for V1405 Cas (in Cassiopeia - the Queen), a recently-discovered nova (a star that brightens suddenly and quite dramatically). As of March 21, it was visible around magnitude +7.6, bright enough to be seen in binoculars. Google it, or go to https://earthsky.org for more information and charts. Novae can dramatically brighten and then, just as dramatically, dim and disappear in a matter of days, so be quick.
Cassiopeia is visible above the northwest horizon by about 8 p.m. this coming week; look for the constellation's distinctive, stretched-out "W" shape.
Until next week, clear skies.
Events:
- March 30 - Moon at perigee (closest to Earth)
- April 4 - Last quarter moon
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers at [email protected].
