Theme: Adjectives deriving from words for celestial bodies.
SATURNINE, MARTIAL, LUNATIC, MERCURIAL, VENEREAL JOVIAL, SUNNY

ACROSS
 7. <taciturn> SATURNINE = S [-m]ATUR[e] NINE
 8. ←S_N _O_E_ + A← = AEONS <vast lengths of time>
10. FLOAT* = ALOFT <into the air>
11. <delivery> CAESARIAN = CANARIES* outside A
12/25. DOG + "DAZE" = DOG DAYS <when Sirius shines at sunrise>
13. (IN SURVEY)* outside ALL = UNIVERSALLY <everywhere>
15. <type of law> MARTIAL = (.u)M ART I AL(l.)
17. <madman> LUNATIC = (IN A CULT)*
20. <Elon Musk, perhaps> ROBBER BARON = *£*ORE BAN*£*
23. <charged particle> ION = N after IO
24. ASHEN outside DAWN* = AS AND WHEN <in due course>
26. <one giving nothing away> MISER = ←RE SIM←
27. <cloudy> MILKY = MY outside ILK
28. <unpredictable> MERCURIAL = MIRACLE* outside UR

DOWN
 1. <more durable> STRONGER = ST ROGER outside _N
 2. <sticking up> PROTRUSIVE = PRO VIRTUES*
 3. KI + MC HI = KIMCHI <Korean side>
 4. V EN E_ + REAL = VENEREAL <relating to it {sex}>
 5. GAMBIA - MB = GAIA <Earth>
 6. <buoyant> JOVIAL = O inside J VIA _L_
 7. ←MOD RATS← STARDOM <celebrity>
 9. <bright> SUNNY = "SONNY"
14. ((MOON + SUN)* + S) outside Y…Y = SYNONYMOUS <coinciding>
16. <dramatic performance> LA BOHEME = LAB 0 HEM + _E
18. <catastrophic fail> TAILSPIN = TAIL S + PIN
19. <power> CONTROL = CON + TROL[-l]
20. <world> REALM = M_ after _REAL
21. <exploratory vessel> BEAGLE <dog>
22. <peter out> RUN DRY = R_ U_ N_ D_ R_ Y_