Earth’s orbit around the sun is really an ellipse, a shape that can be thought of as a “stretched out” circle or an oval.
When Earth is at perihelion, the planet comes closest to the sun. This occurs during winter in the Northern Hemisphere and summer in the Southern. Perihelion will occur on Jan. 4, when the earth is 91,404,322 miles from the sun.
On the other hand, aphelion is when Earth is farthest from the sun and will happen July 3. At that point in time, the Earth will be 94,505,901 miles from the sun, or about a 3 million-mile difference between perihelion and aphelion.
Not only is the Earth’s orbit around the sun an oval, but so is the moon’s orbit around the Earth. The moon is about 30,000 miles closer to Earth at perigee than apogee, when it’s farther away.
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On Nov. 14 the sun, moon and Earth will line up in what is known as syzygy. The moon will also be at perigee; when this happens, astronomers called it perigee-syzygy. Not only will the moon appear brighter and larger (supermoon), but we will see some of the biggest tides of the year (king tides).
The Nov. 14 perigee, or supermoon, will be super special; the last time it appeared this big and bright was 1948. We won’t see the moon look like this again until Nov. 25, 2034. The moon will appear larger when it’s near the horizon and seen through trees or buildings, near mountaintops or by the ocean’s surface. In other words, this effect is an optical illusion, or a “moon illusion.”
The tides also will increase during this period. Tides are enhanced when Earth is at perihelion and the moon at perigee. The gravitational forces exerted by the moon and sun are at their greatest at this time. These “tidal forces” are not the total gravitational forces exerted by the sun and moon on Earth, but the difference between these gravitational forces over the surface of the planet.
This tugging produces a tidal “bulge,” or area of higher sea level on the ocean’s surface.
As Earth rotates eastward on its axis, the Central Coast moves into this bulge, which produces a flood tide and eventually a high tide. As Earth continues to spin, we move into an area of below-normal sea level, or nodes, which produce an ebb tide, eventually reaching low tide. The slack tide is when the tide is not coming in or going out.
Overall, the maximum tidal range along the Central Coast ranges from more than 2 feet below the mean low water mark, to more than 7 feet above it, for a tidal range of more than 9 feet.
On Nov. 15, the high tide will reach 6.7 feet at 9:35 a.m. Later in the day, the low tide will drop to -1.3 feet at Port San Luis.
By the way, the tides along the Central Coast will be a little higher and lower on Dec. 13 and 14.
When looking at the tide tables, remember these are only predicted values using gravitational forces.
During El Niño events, actual tides can be up to 1 foot higher because of thermal expansion of the water column. Storms also can affect sea level. Low atmospheric pressure associated with storms can allow ocean waters to expand, resulting in a temporary increase in sea level. Strong winds can cause additional elevation of sea level because of storm surges.
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PG&E’s 10 employee resource groups — each representing the company’s diverse 20,000-plus workforce — award scholarships annually to help offset the cost of higher education. Scholarship information, including criteria and applications, is available on PG&E’s website. To be considered for a scholarship, all applications must be submitted by Feb. 1. This year, PG&E’s employee resource groups awarded a record $430,000 in scholarships to 176 deserving students. Many recipients are the first in their families to attend college.
John Lindsey is PG&E’s Diablo Canyon Power Plant marine meterologist and a media relations representative. Email him at email@example.com.