The Eta Aquariid Meteor Shower.

The Eta Aquariid Meteor Shower will peak before dawn on Sunday 5 May 2024, with up to 45 meteors per hour at it's peak, radiating from the constellation of Aquarius. The radiant point of this shower does not spend long above the horizon in the Northern Hemisphere at this time of year, but is often a good display in the Southern Hemisphere. The Eta Aquarids are potentially visible between 19 April and 28 May, but are extremely hard to spot away from the peak of activity. With the New Moon falling on 7 May, there should still be some good opportunities for meteor spotting.

The radiant point (point from which the meteors appear to radiate) of the Eta Aquariid Meteors. Universe Today.

Meteor showers are thought to be largely composed of material from the tails of comets. Comets are composed largely of ice (mostly water and carbon dioxide), and when they fall into the inner Solar System the outer layers of this boil away, forming a visible tail (which always points away from the Sun, not in the direction the comet is coming from, as our Earth-bound experience would lead us to expect). Particles of rock and dust from within the comet are freed by this melting (strictly sublimation) of the comet into the tail and continue to orbit in the same path as the comet, falling behind over time.

The Earth passing through a stream of comet dust, resulting in a meteor shower. Not to scale. Astro Bob.

The Eta Aquarid Meteor  Shower is caused by the Earth passing through the trail of Halley's Comet, where it encounters thousands of tiny dust particles shed from the comet as its icy surface is melted (strictly sublimated) by the heat of the Sun. Halley's Comet only visits the inner Solar System every 75 years (most recently in 1986 and next in 2061), but the trail of particles shed by it forms a constant flow, which the Earth crosses twice each year; in May when it causes the Eta Aquarid Meteor Shower and in October when it causes the Orionid Meteor Shower.

Halley's Comet imaged on 8 March 1986 from Easter Island. William Liller/International Halley Watch Large Scale Phenomena Network/NASA/Wikimedia Commons.

Halley's Comet has been observed repeatedly and recognised as the same recurring object since at least 240 BC. However, it takes its modern name from the eighteenth century English Astronomer Edmund Halley, who determined the comet's periodicity in 1705.

Halley's Comet completes one orbit every 75.32 years (27 509 days) on an eccentric, orbit tilted at 162° to the plane of the Solar System (i.e. a retrograde orbit, at an angle of18° to the plane of the Solar System, but travelling in the opposite direction to the majority of the objects in the Solar System), that takes it from 0.56 AU from the Sun (59% of the average distance at which the Earth orbits the Sun, and inside the orbit of the planet Venus) to 35.1 AU from the Sun (35.1 times as far from the Sun as the Earth, and outside the orbit of the planet Neptune). As a comet with a period of more than 20 years but less than 200 years, Halley's Comet is considered to be a Periodic Comet, and a Halley-type Commet.

The orbit of Halley's Comet. Nagual Design/Wikimedia Commons.

Halley's Comet was visited by the European Space Agency's Giotto Probe in and Russian Vega 1 and Vega 2 probes March 1986, which were able to determine that the nucleus of the comet was only 15 km across, although it was surrounded by a coma about 100 000 km in diameter, made up of fragments of dust and ice released from the surface as it was heated by the Sun, causing the ices on its surface to sublimate (turn directly from solids to gasses), and that this material comprised 80% water, 10% carbon monoxide, 2.5% methane and ammonia, as well as trace amounts of more complex hydrocarbons, iron and sodium.

Halley's Comet imaged by the Giotto Probe on 14 March 1986. European Space Agency/Wikimedia Commons.

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Casearia septandra: A new species of tree from southern Guinea.

The genus Casearia comprises fruiting trees in the Willow Family, Salicaceae, found throughout the tropics. There are currently three described species known from West Africa, Casearia inaequalis from Sierra Leone and Côte d'Ivoire, Casearia prismatocarpa, which is found from Sierra Leone, Guinea, and Liberia south to the Congo Basin, and Casearia gambiana, currently known only from The Gambia.

In a paper published in the journal Kew Bulletin on 13 April 2024, Frans Breteler, formerly of the Herbarium Vadense, and Abdoulaye Baldé of the Centre de Formation de Nzérékoré, describe a new species of Casearia from the mountains of southern Guinea. 

The new species is named Casearia septandra, although no explanation is given for this name. It is a tree-forming species reaching about 20 m in height with a trunk diameter at breast height of about 36 cm. Leaves are oval and reach about 3.5 cm in length, flowers are small and white, fruit are orange and up to 1 cm in diameter, spliting into three valves as they ripen.

Casearia septandra. (A) Leafy branchlet; (B) fruits; (C) dehisced fruit. Abdoulaye Baldé & Xander van der Burgt in Breteler & Baldé (2024).

Casearia septandra grows in montane rainforests at altitudes of greater than 900 m above sealevel, in the mountains of southern Guinea. A total of five specimens were located, growing at three separate locations. In addition, some material in a dried herbarium specimen collected in 1949 is considered to belong to the species, although the collection location details are unclear, making it uncertain if this material came from one of the known localities for the species. 

Distribution of Casearia septandra. Breteler & Baldé (2024).

Given the low number of living specimens and the limited geographical distribution of the species, Breteler and Baldé suggest that Casearia septandra be classified as  Endangered under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species, although they note that two of the locations at which Casearia septandra was found are threatened by potentialiron ore mining, and that if this goes ahead the species should be reclassified as Critically Endangered. 

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Islands evacuated after major eruption on Mount Ruang, Indonesia.

The Indonesian Meteorology, Climatology and Geophysics Agency has reported a major series of eruptions on Mount Ruang, an island volcano at the southern tip of the Sangihe Islands, which has prompted the evacuation of both Ruang Island and the nearby Tagulandang Island, more than 12 800 people in total. The volcano erupted three times on Tuesday 30 April 2024, producing ash columns which rose to more than 5 km above sealevel, and drifted west as far as Borneo. Seven airports in Indonesia have been forced to close temporarily due to the dangers of ash from the volcano.

An eruption on Mount Ruang, Indonesia, on 30 April 2024, seen from Tagulandang Island. Endha Reifel Pontoh/Reuters.

The Sangihe Island Arc is a chain of volcanic islands running between the northern tip of Sulawesi and the southern tip of Mindanao. The chain marks the boundary between the Molucca Plate to the east and the Sangihe Plate to the west, with the Molucca Plate being subducted beneath the Sangihe Plate. As the Molucca Plate sinks into the Earth it is heated by the friction and the heat of the planet's interior, causing it to partially melt. Some of the melted material then rises through the overlying Sangihe Plate as magma, fuelling the volcanoes of the Sangihe Arc.

The subduction zones beneath Sulawesi and the surrounding islands. Hall & Spakman (2015).

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Asteroid 2024 HO2 passes the Earth.

Asteroid 2024 HO2 passed by the Earth at a distance of about 37 400 km (0.10 times the average distance between the Earth and the Moon, or 0.025% of the distance between the Earth and the Sun), with a velocity of about 9.244 km per second, slightly before 3.20 pm GMT on Monday 29 April 2024. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2024 HO2 has an estimated equivalent diameter of 2-7 m (i.e. it is estimated that a spherical object with the same volume would be 2-7 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) more than 36 km above the ground, with only fragmentary material reaching the Earth's surface.

The relative positions of 2024 HO2 and the Earth on at 3.00 pm GMT on 29 April  2024. JPL Small Body Database.

2024 HO2 was discovered on 30 April 2024 (the day after its closest approach to the Earth) by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2024 HO2 implies that it was the 64th asteroid (object O2 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended so that A = 1, A1 = 26, A2 = 51, etc., which means that O2 = 14 + (25 x 2) = 64) discovered in the second half of April 2024 (period 2024H - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

2024 HO2 has a 226 day (0.62 year) orbital period, with an elliptical orbit tilted at an angle of 9.18° to the plain of the Solar System which takes in to 0.44 AU from the Sun (44% of the distance at which the Earth orbits the Sun and slightly inside the aphelion distance of Mercury) and out to 1.02 AU (2% further away from the Sun than the Earth). Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2024 HO2 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid.

The relative positions of 2024 HO2 and the planets of the Inner Solar System on at 3.00 pm GMT on 29 April 2024. JPL Small Body Database.

This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in May 2021 and the next predicted in January 2026. 2024 HO2 also has frequent close encounters with the planet Venus, with the last thought to have occurred in August 2000 and the next predicted for October 2032.

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Flash flood kills at least 48 following dam failure in Kenya.

At least 48 people, including 17 minors, have died, dozens more are missing, and more than 110 are being treated in hospital following a dam failure in central Kenya on Monday 29 April 2024. The colonial-era Old Kijabe Dam, near the town of Maai Mahiu in Nakuru County (about 50 km to the northwest of Nairobi) gave way at about 3.00 am local time, following weeks of near-continuous heavy rain in the region, sending water coursing through the village of Kamuchiri, where it swept away people, homes, businesses, vehicles, and livestock.

Volunteers clear debris following a flash flood in Nakuru County, Kenya, on 29 April 2024. Antony Gitonga/Standard.

The incident came amid a series of flooding incidents across East Africa, with more than 160 people known to have died and around 180 000 displaced by flood events in Kenya alone. In neighbouring Tanzania at least 155 people have died and over 200 more have been seriously injured. In Burundi more than 200 000 people have been forced to leave their homes due to flooding, and deaths have also been reported in Rwanda and Ethiopia.

Flooding in the village of Mororo in Garissa County, Kenya. Andrew Kasuku/AP.

East Africa typically has two rainy seasons, one lasting from March to May and the other from October to December. This year the early rains have been particularly severe, driven by a combination of a strong El Niño system and a Positive Indian Ocean Dipole phase, both of which are in turn fuelled by this years exceptionally strong global temperatures, which are a result of the rising levels of greenhouse gasses (carbon dioxide, methane, and water) within the Earth's atmosphere, a direct result of Human activities (although the entire continent of Africa is thought to be responsible for only 2-3% of all anthropogenic greenhouse gas emissions).

The El Niño is the warm phase of a long-term climatic oscillation affecting the southern Pacific, which can influence the climate around the world. The onset of El Niño conditions is marked by a sharp rise in temperature and pressure over the southern Indian Ocean, which then moves eastward over the southern Pacific. This pulls rainfall with it, leading to higher rainfall over the Pacific and lower rainfall over South Asia. This reduced rainfall during the already hot and dry summer leads to soaring temperatures in southern Asia, followed by a rise in rainfall that often causes flooding in the Americas and sometimes Africa. Worryingly climatic predictions for the next century suggest that global warming could lead to more frequent and severe El Niño conditions, extreme weather conditions a common occurrence.

Movements of air masses and changes in precipitation in an El Niño weather system. Fiona Martin/NOAA.

Indian Ocean Dipole Phases are similar to the El Niño/La Niña climatic oscillation that affect the Pacific Ocean. Under normal circumstances equatorial waters off the east coast of Africa and west coast of Indonesia are roughly similar in temperature, however during a Positive Indian Ocean Dipole Phase the waters off the coast of Africa become significantly warmer. As the prevailing currents in the area flow west to east, this warm water is then pushed onto the shallower continental shelf off the East African coast, where it warms the air over the sea more rapidly, leading to increased evaporation (which fuels rain) and a drop in air pressure over the western Indian Ocean. This in turn drives air currents over the Indian Ocean to flow more strongly east to west, leading to higher rates of  warming off the coast of Africa and more cooling off the coast of Indonesia, fuelling a feedback cycle that tends to remain through the winter season in any year when it forms. This leads to a particularly wet rainy season across much of East Africa, while much of Australia is at risk of drought (during a Negative Indian Ocean Dipole Phase the reverse happens, with drought in East Africa and flooding in Australia).

Areas of warming and cooling and air flow during a Positive Indian Ocean Dipole Phase. NOAA.

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