APOD: Perihelion to Aphelion (2021 Jul 08)

[APOD Robot]

Perihelion to Aphelion

Explanation: Aphelion for 2021 occurred on July 5th. That's the point in Earth's elliptical orbit when it is farthest from the Sun. Of course, the distance from the Sun doesn't determine the seasons. Those are governed by the tilt of Earth's axis of rotation, so July is still summer in the north and winter in the southern hemisphere. But it does mean that on July 5 the Sun was at its smallest apparent size when viewed from planet Earth. This composite neatly compares two pictures of the Sun, both taken with the same telescope and camera. The left half was captured close to the date of the 2021 perihelion (January 2), the closest point in Earth's orbit. The right was recorded just before the aphelion in 2021. Otherwise difficult to notice, the change in the Sun's apparent diameter between perihelion and aphelion amounts to a little over 3 percent.

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[FrankTKO]

Are perihelion and aphelion always on the same date and time?

[Chris Peterson]

Are perihelion and aphelion always on the same date and time?

Close, but not exactly. The barycenter of the Earth-Moon system is actually what is in a stable orbit around the Sun. The Earth is sort of wobbling around that, which shifts the actual dates of the apses slightly- about a day either way. Over the longer term, the precession of the Earth changes the dates, cycling through a full year every 25,000 years or so.

[VictorBorun]

the precession of the Earth changes the dates, cycling through a full year every 25,000 years or so.

The Earth's apsidal precession slowly increases its argument of periapsis; it takes about 112,000 years for the ellipse to revolve once relative to the fixed stars. The Earth's polar axis, and hence the solstices and equinoxes, precess with a period of about 26,000 years in relation to the fixed stars. These two forms of 'precession' combine so that it takes between 20,800 and 29,000 years for the ellipse to revolve once relative to the vernal equinox.

[JohnD]

So January 4th, the next perihelion we will have a SUPERSUN! 'Cos it will be 3% bigger!!!!
Break out the whoowhoo flags, invent new names for these aspects, just like all the flimflam about the Moon that is so boring.

Nice pics!

[isoparix]

'Of course, the distance from the Sun doesn't determine the seasons.' Of course not.
But 3% further away is 3% more distant and therefore surely , means (1/1.03)^2, or approx 6%, less insolation? And a difference of 6% between min and max energy received ought to have some traceable impact on the weather?

[orin stepanek]

Well I'm glad Aphelion comes in July, Might not mean much, but
it's hot enough in the summer!

[Sa Ji Tario]

Isoparix, Although it is closer to the sun in the summer of the southern hemisphere, the difference in energy received is moderated with the larger illuminated water surface

[johnnydeep]

Isoparix, Although it is closer to the sun in the summer of the southern hemisphere, the difference in energy received is moderated with the larger illuminated water surface

At first I thought isoparix was some astronomical term. Then I scrolled back to find that that was the name of the poster to whom you were directing your reply. Note: in case you didn't know, if you click the big double quote icon/button on someone's post, you can quote it with attribution so as to make the conversation more clear.

[FrankTKO]

Are perihelion and aphelion always on the same date and time?

Close, ...

Thanks!

[Chris Peterson]

Isoparix, Although it is closer to the sun in the summer of the southern hemisphere, the difference in energy received is moderated with the larger illuminated water surface

To be clear, the difference in energy isn't moderated. The effect on climate created by that difference is moderated by the oceans.

We should also note that the Earth is moving faster in its orbit at perihelion than aphelion, which makes the northern hemisphere summer a little longer than the southern hemisphere summer. As with the distance variation, this produces a measurable, although tiny impact on climate.

[neufer]

'Of course, the distance from the Sun doesn't determine the seasons.' Of course not.

But 3% further away is 3% more distant and therefore surely , means (1/1.03)^2, or approx 6%, less insolation? And a difference of 6% between min and max energy received ought to have some traceable impact on the weather?

Although it is closer to the sun in the summer of the southern hemisphere, the difference in energy received is moderated with the larger illuminated water surface.

6% of 240 W/m² of solar energy input to the Earth's energy budget is 15 W/m²

Tropic of Capricorn ocean expanse only exacerbates this annual modulation of the solar energy input to the Earth's energy budget:

• Reflective Tropic of Cancer deserts have noticeably higher clear sky albedo than dark Tropic of Capricorn oceans to modulate the ~23 W/m² is reflected by the surface albedo. Those Tropic of Cancer deserts will also more effectively radiate off their absorbed heat into space rather than absorbing the radiation and/or transforming it into warm moist air.

<<Of the ~340 W/m² of solar radiation received by the Earth, an average of ~77 W/m2 is reflected back to space by clouds and the atmosphere and ~23 W/m² is reflected by the surface albedo, leaving ~240 W/m² of solar energy input to the Earth's energy budget. This gives the Earth a mean net albedo (specifically, its Bond albedo) of 0.306.>>

[Sa Ji Tario]

Thank you so much Johnnydeep I've learned something else

[MarkBour]

'Of course, the distance from the Sun doesn't determine the seasons.' Of course not.
But 3% further away is 3% more distant and therefore surely , means (1/1.03)^2, or approx 6%, less insolation? And a difference of 6% between min and max energy received ought to have some traceable impact on the weather?

Isoparix, Although it is closer to the sun in the summer of the southern hemisphere, the difference in energy received is moderated with the larger illuminated water surface

>>>>Wikipedia https://en.wikipedia.org/wiki/Apsis#Per ... d_aphelion<<<<
Because of the increased distance at aphelion, only 93.55% of the radiation from the Sun falls on a given area of Earth's surface as does at perihelion, but this does not account for the seasons, which result instead from the tilt of Earth's axis of 23.4° away from perpendicular to the plane of Earth's orbit. Indeed, at both perihelion and aphelion it is summer in one hemisphere while it is winter in the other one. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, regardless of the Earth's distance from the Sun.

In the northern hemisphere, summer occurs at the same time as aphelion, when solar radiation is lowest. Despite this, summers in the northern hemisphere are on average 2.3 °C (4 °F) warmer than in the southern hemisphere, because the northern hemisphere contains larger land masses, which are easier to heat than the seas.

The above Wikipedia article references the following, which has a bit more detail.
[The following includes quotes of: Roy Spencer of NASA's Global Hydrology and Climate Center (GHCC).]

>>>>https://spaceweather.com/glossary/aphelion.html<<<<
But there's more to the story: Says Spencer, "the average temperature of the whole earth at aphelion is about 4°F or 2.3°C higher than it is at perihelion." Our planet is actually warmer when we're farther from the Sun!



Above: Earth's land-masses are concentrated more north of the equator than south. Image credit and copyright: the PALEOMAP Project.

This happens because continents and oceans aren't distributed evenly around the globe. There's more land in the northern hemisphere and more water in the south. During the month of July the land-crowded northern half of our planet is tilted toward the Sun. "Earth's temperature is slightly higher in July because the Sun is shining down on all that land, which heats up rather easily," says Spencer.

Physicists would say that continents have low heat capacity. "Consider the desert," says Bill Patzert, an oceanographer at NASA's Jet Propulsion Laboratory. "At night the desert is cold, perhaps only 60° F (16° C). When the Sun rises in the morning the temperature might jump to 100° F (38° C) or more." Such mercurial behavior is characteristic of materials like rocks and soil with low heat capacity. It doesn't take much sunlight to substantially elevate their temperature.

Water is different. It has high heat capacity. "Let's say you went sailing off Malibu Beach at noon," continues Patzert. "The offshore temperature might be 75° F (24° C) -- pretty pleasant!" What happens after sunset? "The temperature drops, but only a few degrees because the heat capacity of the ocean is so high."

All this explains why July is our planet's warmest month: Northern continents baked by the aphelion Sun elevate the average temperature of the entire globe. January, on the other hand, is the coolest month because that's when our planet presents its water-dominated hemisphere to the Sun. "We're closer to the Sun in January," says Spencer, "but the extra sunlight gets spread throughout the oceans." Southern summer in January (perihelion) is therefore cooler than northern summer in July (aphelion).

I'm not sure I fully understand the data, statements, and reasoning in this last part. What it seems to be saying is:

If you measure temperature of the air about 2 meters off of the surface, and if you sample that all around the globe on a given date and average the numbers, then you'll find a higher average in July than in January. This is in spite of the fact that the "heat budget" in July begins with less solar radiation hitting the planet than in January. And they're saying that the radiation is heating more water in July, raising it only x degrees, say, on average, whereas in January, more radiation hits terra firma and the temperature above such land jumps a bunch more than x degrees.

----------

A detailed view would perhaps be done by dividing the Earth's surface into a massive number of small cells and observing the heat exchange going on in each unit over time. And then you'd want to include heat exchange from one cell to its neighbors as well, and weather and ocean currents might need to come into play -- but maybe you could do a good representation of the overall effect we're discussing, without all of that.

I assume someone (very likely Dr. Spencer) has already done this, since it sounds like just the very beginning of any decent weather model.

But what I find interesting here is that in January, the Earth is in some sense almost certainly "warmer" than it is in July, but the average air temperature felt by surface dwellers is lower in January, and that's not just because most of us live in the northern hemisphere, it's actually a true average over all points that are 2 meters off of the surface of the globe.

At least that's how I'm reading this.

[Chris Peterson]

But what I find interesting here is that in January, the Earth is in some sense almost certainly "warmer" than it is in July, but the average air temperature felt by surface dwellers is lower in January, and that's not just because most of us live in the northern hemisphere, it's actually a true average over all points that are 2 meters off of the surface of the globe.

At least that's how I'm reading this.

That's completely believable. Consider the mesosphere, which is coldest in summer months. The Earth's atmosphere is complex thermodynamic engine, and adding energy to it can either result in an increase or decrease in temperature, depending on where and when you measure it.

[neufer]

Consider the mesosphere, which is coldest in summer months. The Earth's atmosphere is complex thermodynamic engine, and adding energy to it can either result in an increase or decrease in temperature, depending on where and when you measure it.

Click to play embedded YouTube video.

<<The mesopause (not to be confused with menopause) is the point of minimum temperature at the boundary between the mesosphere and the thermosphere atmospheric regions. Due to the lack of solar heating and very strong radiative cooling from carbon dioxide, the mesosphere is the coldest region on Earth with temperatures as low as -100 °C (173 K). The altitude of the mesopause for many years was assumed to be at around 85 km, but observations to higher altitudes and modeling studies in the last 10 years have shown that in fact the mesopause consists of two minima - one at about 85 km and a stronger minimum at about 100 km.

Another feature is that the summer mesopause is cooler than the winter (sometimes referred to as the mesopause anomaly). It is due to a summer-to-winter circulation giving rise to upwelling at the summer pole and downwelling at the winter pole. Air rising will expand and cool resulting in a cold summer mesopause and conversely downwelling air results in compression and associated increase in temperature at the winter mesopause. In the mesosphere the summer-to-winter circulation is due to gravity wave dissipation, which deposits momentum against the mean east–west flow, resulting in a small north–south circulation.

In recent years the mesopause has also been the focus of studies on global climate change associated with increases in CO₂. Unlike the troposphere, where greenhouse gases result in the atmosphere heating up, increased CO₂ in the mesosphere acts to cool the atmosphere due to increased radiative emission. This results in a measurable effect - the mesopause should become cooler with increased CO₂. Observations do show a decrease of temperature of the mesopause, though the magnitude of this decrease varies and is subject to further study.>>