A solar eclipse occurs when the Moon passes between Earth and the Sun

 A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season in its new moon phase, when the Moon's orbital plane is closest to the plane of the Earth's orbit.^[1] In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured.

 Unlike a lunar eclipse, which may be viewed from anywhere on the night side of Earth, a solar eclipse can only be viewed from a relatively small area of the world. As such, although total solar eclipses occur somewhere on Earth every 18 months on average, they recur at any given place only once every 360 to 410 years.

If the Moon were in a perfectly circular orbit and in the same orbital plane as Earth, there would be total solar eclipses once a month, at every new moon. Instead, because the Moon's orbit is tilted at about 5 degrees to Earth's orbit, its shadow usually misses Earth. Solar (and lunar) eclipses therefore happen only during eclipse seasons, resulting in at least two, and up to five, solar eclipses each year, no more than two of which can be total.^[2][3] Total eclipses are more rare because they require a more precise alignment between the centers of the Sun and Moon, and because the Moon's apparent size in the sky is sometimes too small to fully cover the Sun.

An eclipse is a natural phenomenon. In some ancient and modern cultures, solar eclipses were attributed to supernatural causes or regarded as bad omens. Astronomers' predictions of eclipses began in China as early as the 4th century BC; eclipses hundreds of years into the future may now be predicted with high accuracy.

Looking directly at the Sun can lead to permanent eye damage, so special eye protection or indirect viewing techniques are used when viewing a solar eclipse. Only the total phase of a total solar eclipse is safe to view without protection. Enthusiasts known as eclipse chasers or umbraphiles travel to remote locations to see solar eclipses.^[4][5]

^{Central eclipse is often used as a generic term for a total, annular, or hybrid eclipse.[15] This is, however, not completely correct: the definition of a central eclipse is an eclipse during which the central line of the umbra touches the Earth's surface. It is possible, though extremely rare, that part of the umbra intersects with the Earth (thus creating an annular or total eclipse), but not its central line. This is then called a non-central total or annular eclipse.[15] Gamma is a measure of how centrally the shadow strikes. The last (umbral yet) non-central solar eclipse was on April 29, 2014. This was an annular eclipse. The next non-central total solar eclipse will be on April 9, 2043.[16]}

^{#SolarEclipseWonders #CelestialSpectacle #TotalityMagic #EclipseChasers}

 ^{#SunMoonDance #AstronomyLovers #EclipseEnchantment}

 ^{#DarknessToLight #EclipseAdventures #StellarExperiences}

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Doppler effect

 The Doppler effect is a phenomenon observed in waves, such as sound waves and light waves, where the frequency or wavelength of the wave appears to change when the source of the wave and the observer are in relative motion. This effect is named after the Austrian physicist Christian Doppler, who first described it in 1842.

A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.^[4]

The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave.^[4][5] Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wave fronts is reduced, so the waves "bunch together". Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wave fronts is then increased, so the waves "spread out".

For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted.^[3] The total Doppler effect may therefore result from motion of the source, motion of the observer, motion of the medium, or any combination thereof. For waves propagating in vacuum, such as electromagnetic waves or gravitational waves, only the difference in velocity between the observer and the source needs to be considered. If this relative speed is not negligible compared to the speed of light, a more complicated relativistic Doppler effect arises.

In classical physics, where the speeds of source and the receiver relative to the medium are lower than the speed of waves in the medium, the relationship between observed frequency ${\displaystyle �}$ and emitted frequency ${\displaystyle {�}_{\text{0}}}$ is given by:^[8]

$${\displaystyle �=\left(\frac{�\pm {�}_{\text{r}}}{�\pm {�}_{\text{s}}}\right){�}_0}$$

where

• ${\displaystyle �}$ is the propagation speed of waves in the medium;
• ${\displaystyle {�}_{\text{r}}}$ is the speed of the receiver relative to the medium, added to ${\displaystyle �}$ if the receiver is moving towards the source, subtracted if the receiver is moving away from the source;
• ${\displaystyle {�}_{\text{s}}}$ is the speed of the source relative to the medium, added to ${\displaystyle �}$ if the source is moving away from the receiver, subtracted if the source is moving towards the receiver.

Note this relationship predicts that the frequency will decrease if either source or receiver is moving away from the other.

Equivalently, under the assumption that the source is either directly approaching or receding from the observer:

$${\displaystyle \frac{�}{{�}_{��}}=\frac{{�}_0}{{�}_{��}}=\frac{1}{�}}$$

where

• ${\displaystyle {�}_{��}}$ is the wave's speed relative to the receiver;
• ${\displaystyle {�}_{��}}$ is the wave's speed relative to the source;
• ${\displaystyle �}$ is the wavelength.

If the source approaches the observer at an angle (but still with a constant speed), the observed frequency that is first heard is higher than the object's emitted frequency. Thereafter, there is a monotonic decrease in the observed frequency as it gets closer to the observer, through equality when it is coming from a direction perpendicular to the relative motion (and was emitted at the point of closest approach; but when the wave is received, the source and observer will no longer be at their closest), and a continued monotonic decrease as it recedes from the observer. When the observer is very close to the path of the object, the transition from high to low frequency is very abrupt. When the observer is far from the path of the object, the transition from high to low frequency is gradual.

If the speeds ${\displaystyle {�}_{\text{s}}}$ and ${\displaystyle {�}_{\text{r}}}$ are small compared to the speed of the wave, the relationship between observed frequency ${\displaystyle �}$ and emitted frequency ${\displaystyle {�}_{\text{0}}}$ is approximately^[8]

Observed frequency	Change in frequency
${\displaystyle �=\left(1+\frac{\mathrm{\Delta }�}{�}\right){�}_0}$	${\displaystyle \mathrm{\Delta }�=\frac{\mathrm{\Delta }�}{�}{�}_0}$

where

• ${\displaystyle \mathrm{\Delta }�=�-{�}_0}$
• ${\displaystyle \mathrm{\Delta }�=-({�}_{\text{r}}-{�}_{\text{s}})}$ is the opposite of the relative speed of the receiver with respect to the source: it is positive when the source and the receiver are moving towards each other.

universe.
#DopplerEffect
#Physics
#Science
#SoundWaves
#LightWaves
#Redshift

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Aditya L1 launch: ISRO to perform first Earth-bound firing today

 

Aditya-L1 mission: Aditya-L1 will stay Earth-bound orbits for 16 days, during which it will undergo five manoeuvres to gain necessary velocity for its journey.

The Indian Space Research Organisation (ISRO) said the first Earth-bound firing to raise Aditya-L1's orbit is scheduled at around 11:45 am on Sunday, a day after the Polar Satellite Launch Vehicle or PSLV-C57.1 rocket carrying the orbiter lifted off successfully from the Satish Dhawan Space Centre in Andhra Pradesh's Sriharikota.

The successful launch of ISRO’s first solar mission came a week after its historic lunar landing mission — Chandrayaan-3.

Top updates on Aditya-L1 solar mission

1. "Aditya-L1 started generating the power. The solar panels are deployed. The first Earth-Bound firing to raise the orbit is scheduled for September 3 around 11:45 hours," the ISRO said on Saturday.

Aditya-L1 started generating the power. The solar panels are deployed. The first EarthBound firing to raise the orbit is scheduled for September 3, 2023, around 11:45 Hrs. IST

2. The Earth-bound manoeuvres will involve the rockets firing and some adjustments to angles, as required. How this will work can perhaps be understood by taking the example of when a person is on a swing — to make the swing go higher, a pressure (by shifting body weight) is applied when in the phase when the swing is coming down towards the ground. In Aditya-L1’s case, once it gains enough velocity, it will slingshot around to its intended path towards L1.

3. The PSLV has placed the Aditya-L1 satellite precisely into its intended orbit, the agency said.

4. Aditya-L1 will stay Earth-bound orbits for 16 days, during which it will undergo five maneuvres to gain the necessary velocity for its journey, the ISRO said.

5. Subsequently, Aditya-L1 will undergo a trans-Lagrangian1 insertion manoeuvre, marking the beginning of its 110-day trajectory to the destination around the L1 Lagrange Point, it said.

6. Once arrived at the L1 point, another manoeuvre will bind Aditya-L1 to an orbit around L1, a balanced gravitational location between the Earth and the Sun. The satellite will spend its whole mission life orbiting around L1 in an irregularly shaped orbit in a plane roughly perpendicular to the line joining the Earth and the Sun.

7. According to the agency, the Aditya-L1 mission is expected to reach the observation point in four months. It will be placed in a halo orbit around Lagrangian Point 1 (or L1), which is 1.5 million km away from the Earth in the direction of the Sun.

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Dark Storm on Neptune Reverses Direction, Possibly Shedding a Fragment

 The storm, which is wider than the Atlantic Ocean, was born in the planet's northern hemisphere and discovered by Hubble in 2018. Observations a year later showed that it began drifting southward toward the equator, where such storms are expected to vanish from sight. To the surprise of observers, Hubble spotted the vortex change direction by August 2020, doubling back to the north. Though Hubble has tracked similar dark spots over the past 30 years, this unpredictable atmospheric behavior is something new to see.

Equally as puzzling, the storm was not alone. Hubble spotted another, smaller dark spot in January this year that temporarily appeared near its larger cousin. It might possibly have been a piece of the giant vortex that broke off, drifted away, and then disappeared in subsequent observations.

"We are excited about these observations because this smaller dark fragment is potentially part of the dark spot’s disruption process," said Michael H. Wong of the University of California at Berkeley. "This is a process that's never been observed. We have seen some other dark spots fading away, and they're gone, but we've never seen anything disrupt, even though it’s predicted in computer simulations."

The large storm, which is 4,600 miles across, is the fourth dark spot Hubble has observed on Neptune since 1993. Two other dark storms were discovered by the Voyager 2 spacecraft in 1989 as it flew by the distant planet, but they had disappeared before Hubble could observe them. Since then, only Hubble has had the sharpness and sensitivity in visible light to track these elusive features, which have sequentially appeared and then faded away over a duration of about two years each. Hubble uncovered this latest storm in September 2018.

Wicked Weather

Neptune's dark vortices are high-pressure systems that can form at mid-latitudes and may then migrate toward the equator. They start out remaining stable due to Coriolis forces, which cause northern hemisphere storms to rotate clockwise, due to the planet's rotation. (These storms are unlike hurricanes on Earth, which rotate counterclockwise because they are low-pressure systems.) However, as a storm drifts toward the equator, the Coriolis effect weakens and the storm disintegrates. In computer simulations by several different teams, these storms follow a more-or-less straight path to the equator, until there is no Coriolis effect to hold them together. Unlike the simulations, the latest giant storm didn't migrate into the equatorial "kill zone."

"It was really exciting to see this one act like it's supposed to act and then all of a sudden it just stops and swings back," Wong said. "That was surprising.

Dark Spot Jr.

The Hubble observations also revealed that the dark vortex’s puzzling path reversal occurred at the same time that a new spot, informally deemed "dark spot jr.," appeared. The newest spot was slightly smaller than its cousin, measuring about 3,900 miles across. It was near the side of the main dark spot that faces the equator — the location that some simulations show a disruption would occur.

However, the timing of the smaller spot's emergence was unusual. "When I first saw the small spot, I thought the bigger one was being disrupted," Wong said. "I didn't think another vortex was forming because the small one is farther towards the equator. So it's within this unstable region. But we can't prove the two are related. It remains a complete mystery.

"It was also in January that the dark vortex stopped its motion and started moving northward again," Wong added. "Maybe by shedding that fragment, that was enough to stop it from moving towards the equator."

The researchers are continuing to analyze more data to determine whether remnants of dark spot jr. persisted through the rest of 2020.

Dark Storms Still Puzzling

It's still a mystery how these storms form, but this latest giant dark vortex is the best studied so far. The storm's dark appearance may be due to an elevated dark cloud layer, and it could be telling astronomers about the storm's vertical structure.

Another unusual feature of the dark spot is the absence of bright companion clouds around it, which were present in Hubble images taken when the vortex was discovered in 2018. Apparently, the clouds disappeared when the vortex halted its southward journey. The bright clouds form when the flow of air is perturbed and diverted upward over the vortex, causing gases to likely freeze into methane ice crystals. The lack of clouds could be revealing information on how spots evolve, say researchers.

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NASA Selects Geology Team for the First Crewed Artemis Lunar Landing

 NASA has selected the geology team that will develop the surface science plan for the first crewed lunar landing mission in more than 50 years. NASA’s Artemis III mission will land astronauts, including the first woman to land on the Moon, near the lunar South Pole to advance scientific discovery and pave the way for long-term lunar exploration.

“Science is one of the pillars of Artemis,” said Dr. Nicky Fox, NASA Science Associate Administrator. “This team will be responsible for leading the geology planning for humanity’s first return to the lunar surface in more than 50 years, ensuring that we maximize the science return of Artemis and grow in our understanding of our nearest celestial neighbor.”

The Artemis III Geology Team, led by principal investigator Dr. Brett Denevi of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, will work with the agency to determine the mission’s geological science objectives and design the geology surface campaign that the Artemis astronauts will carry out on the Moon during this historic mission. These objectives will be defined in accordance with the established Artemis science priorities.

“Selecting this team marks an important step in our efforts to optimize the science return of Artemis III. This team of well-respected lunar scientists has demonstrated experience with science operations, sample analysis, and operational flexibility, all of which is critical for the successful incorporation of science during Artemis III,” said Dr. Joel Kearns, deputy associate administrator for exploration in NASA’s Science Mission Directorate at NASA Headquarters in Washington. “With the establishment of the Artemis III Geology Team, we are ensuring that NASA will build a strong lunar science program.”

The other co-investigators on the Artemis III Geology Team are:

The Geology Team’s focus will be to plan the Artemis III astronauts’ science activities during their moonwalks, which will include field geology traverses, observations, and the collection of lunar samples, imagery, and scientific measurements. The team will also support the real-time documentation and initial assessment of scientific data during astronaut lunar operations. Members will then evaluate the data returned by the mission, including preliminary examination and cataloguing of the first lunar samples collected by NASA since 1972.

"The Artemis III Geology Team will have the unique opportunity to analyze the first-ever samples from the lunar south pole region, helping us not only to unlock new information about the formation of our Solar System, but also with planning for future Artemis missions and establishing a long-term lunar presence,” said Jim Free, Associate Administrator for NASA’s Exploration Systems Mission Directorate.

The collection of samples and data from this region, which contains some of the oldest parts of the Moon, estimated to be at least 3.85 billion years old, will help scientists better understand fundamental planetary processes that operate across the solar system and beyond. The resulting analysis from the geology team’s activities could also help yield important information about the depth, distribution, and composition of ice at the Moon’s South Pole. This information is valuable from both a scientific and a resource perspective because oxygen and hydrogen can be extracted from lunar ice to be used for life support systems and fuel.

The team, which was chosen through a dual-anonymous peer review process, will have a budget of $5.1 million to lead the geology for Artemis III.

The members of this geology team are part of the broader Artemis Science Team and will work in coordination with Artemis III Project Scientist, Dr. Noah Petro, and the NASA Artemis Internal Science Team, as well as participating scientists, and deployed payload teams that will be selected from future or ongoing competitive solicitations.

Through Artemis, NASA will land the first woman and first person of color on the Moon, establishing a long-term, sustainable lunar presence to explore more of the lunar surface than ever before and prepare for future astronaut missions to Mars.

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